Improve syntax highlighting colors (#4572)
Improve syntax highlighting colors (#4572)

- Highlight the error location markers in bold red.
- Dim the line number gutter with grey.
- Don't highlight brackets. Few editor color schemes do.
- Add JSX tag highlighting.
- Don't highlight punctuators with bold. That looks bad on Ubuntu based
  systems. Instead, highlight them the same way as JSX tags, which
  results in really nice JSX highlighting.
- Highlight capitalized variable names.
- Make invalid tokens stand out with a red background.
hzoo/babel,guybedford/babel,shuhei/babel,zjmiller/babel,hzoo/babel,PolymerLabs/babel,kaicataldo/babel,ccschneidr/babel,jridgewell/babel,garyjN7/babel,kellyselden/babel,kaicataldo/babel,hulkish/babel,chicoxyzzy/babel,babel/babel,jridgewell/babel,STRML/babel,existentialism/babel,maurobringolf/babel,zertosh/babel,kassens/babel,chicoxyzzy/babel,kellyselden/babel,jridgewell/babel,claudiopro/babel,claudiopro/babel,Skillupco/babel,rmacklin/babel,iamchenxin/babel,iamchenxin/babel,kellyselden/babel,zjmiller/babel,kaicataldo/babel,kedromelon/babel,hulkish/babel,babel/babel,PolymerLabs/babel,shuhei/babel,guybedford/babel,bcoe/babel,Skillupco/babel,babel/babel,babel/babel,shuhei/babel,kellyselden/babel,KunGha/babel,tikotzky/babel,ccschneidr/babel,kedromelon/babel,existentialism/babel,KunGha/babel,hulkish/babel,chicoxyzzy/babel,kaicataldo/babel,maurobringolf/babel,existentialism/babel,Skillupco/babel,hulkish/babel,PolymerLabs/babel,tikotzky/babel,claudiopro/babel,kedromelon/babel,samwgoldman/babel,rmacklin/babel,jridgewell/babel,STRML/babel,zertosh/babel,bcoe/babel,Skillupco/babel,garyjN7/babel,vadzim/babel,vadzim/babel,chicoxyzzy/babel,iamchenxin/babel,hzoo/babel,samwgoldman/babel,hzoo/babel,guybedford/babel,samwgoldman/babel,kassens/babel,maurobringolf/babel