Migrate to eslint

Summary
We'd like contributors to use the same linter and lint rules that we use
internally.  This diff swaps out eslint for jshint and fixes all lint failures
except for the max-len failures in the test suites.

Test Plan:
- ka-lint src
- make lint
- make test

Reviewers: emily

.eslintrc

@@ -0,0 +1,86 @@
+{
+    "rules": {
+        "arrow-spacing": 2,
+        "brace-style": [2, "1tbs", { "allowSingleLine": true }],
+        // We'd possibly like to remove the 'properties': 'never' one day.
+        "camelcase": [2, { "properties": "never" }],
+        "comma-dangle": [2, "always-multiline"],
+        "comma-spacing": [2, { "before": false, "after": true }],
+        "constructor-super": 2,
+        "curly": 2,
+        "eol-last": 2,
+        "eqeqeq": [2, "allow-null"],
+        "guard-for-in": 2,
+        "indent": [2, 4, {"SwitchCase": 1}],
+        "linebreak-style": [2, "unix"],
+        "max-len": [2, 80, 4, { "ignoreUrls": true, "ignorePattern": "\\brequire\\([\"']|eslint-disable" }],
+        "no-alert": 2,
+        "no-array-constructor": 2,
+        "no-console": 2,
+        "no-const-assign": 2,
+        "no-debugger": 2,
+        "no-dupe-class-members": 2,
+        "no-dupe-keys": 2,
+        "no-extra-bind": 2,
+        "no-new": 2,
+        "no-new-func": 2,
+        "no-new-object": 2,
+        "no-spaced-func": 2,
+        "no-this-before-super": 2,
+        "no-throw-literal": 2,
+        "no-trailing-spaces": 2,
+        "no-undef": 2,
+        "no-unexpected-multiline": 2,
+        "no-unreachable": 2,
+        "no-unused-vars": [2, {"args": "none", "varsIgnorePattern": "^_*$"}],
+        "no-useless-call": 2,
+        "no-with": 2,
+        "one-var": [2, "never"],
+        "prefer-const": 2,
+        "prefer-spread": 2,
+        "semi": [2, "always"],
+        "space-after-keywords": 2,
+        "space-before-blocks": 2,
+        "space-before-function-paren": [2, "never"],
+        "space-before-keywords": 2,
+        "space-infix-ops": 2,
+        "space-return-throw-case": 2,
+        "space-unary-ops": 2,
+        // ---------------------------------------
+        // Stuff we explicitly disable.
+        // We turned this off because it complains when you have a
+        // multi-line string, which I think is going too far.
+        "prefer-template": 0,
+        // We've decided explicitly not to care about this.
+        "arrow-parens": 0,
+        // ---------------------------------------
+        // Stuff that's disabled for now, but maybe shouldn't be.
+        // disabled because KaTeX doesn't use ES6
+        "no-var": 0,
+        // TODO(csilvers): enable these if/when community agrees on it.
+        "prefer-arrow-callback": 0,
+        "object-curly-spacing": [0, "always"],
+        // Might be nice to turn this on one day, but since we don't
+        // use jsdoc anywhere it seems silly to require it yet.
+        "valid-jsdoc": 0,
+        "require-jsdoc": 0
+    },
+    "ecmaFeatures": {
+        "arrowFunctions": true,
+        "blockBindings": true,
+        "classes": true,
+        "destructuring": true,
+        "experimentalObjectRestSpread": true,
+        "forOf": true,
+        "jsx": true,
+        "restParams": true,
+        "spread": true,
+        "templateStrings": true
+    },
+    "env": {
+        // "es6": true,
+        "node": true,
+        "browser": true
+    },
+    "extends": "eslint:recommended"
+}