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

Makefile

@@ -39,6 +39,12 @@ build/fonts:
 		cp static/fonts/$$font* $@; \
 	done
 
+test/screenshotter/unicode-fonts:
+	git clone https://github.com/Khan/KaTeX-test-fonts test/screenshotter/unicode-fonts
+	cd test/screenshotter/unicode-fonts && \
+	git checkout 99fa66a2da643218754c8236b9f9151cac71ba7c && \
+	cd ../../../
+
 contrib: build/contrib
 
 .PHONY: build/contrib
@@ -90,5 +96,5 @@ extended_metrics:
 clean:
 	rm -rf build/*
 
-screenshots:
+screenshots: test/screenshotter/unicode-fonts
 	dockers/Screenshotter/screenshotter.sh