Tokenize let and var as storage.type.var.js

[winstliu]

#206 is back! 🎉

Tested my changes locally and I think I can get everything working this time (it's hard to tell because I get ~50 spec failures on atom/atom). Will be pushing PRs to language-html and atom shortly.

/cc @atom/languages @slinto

Actually fixes #201

[jerone]

Shouldn't const be included in that list of variable declaration types too?

Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements#Declarations
Ref: http://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations

[winstliu]

Maybe not storage.type.var...does storage.type.constant sound better? /cc @MaximSokolov

[MaximSokolov]

storage.type.const or just storage.type as styles for constant can affect "constant" keyword
(see atom/language-go#63 (comment))

Related question: Is const in const int i; a "modifier" or it's storage.type as well?