Add support for Latin-1, Cyrillic, and CJK characters inside \text{} (#508)

Summary:
This diff provides support for Latin-1, Cyrillic, and CJK characters
inside \text{} groups.  For Latin-1 and Cyrillic characters we use
glyph metrics from a glyph from Basic Latin that has roughly the same
bounding box.  We use the metrics for a capital 'M' to approximate the
full-width CJK characters.  Half-width characters are not supported yet.

Test Plan:
- make test
- make screenshots

Reviewers: emily

.travis.yml

@@ -10,4 +10,5 @@ before_script:
   - docker images --no-trunc
 script:
   - npm test
+  - git clone https://github.com/Khan/KaTeX-test-fonts test/screenshotter/unicode-fonts
   - dockers/Screenshotter/screenshotter.sh --verify