Add support for comments

[jaredlockhart]

It'd be helpful to have a syntax for comments in JEXL strings like:

// This is a comment
"this is a literal"

Right now there doesn't seem to be any way to add a comment to a JEXL statement:

In [6]: list(jexl.validate("true"))
Out[6]: []

In [7]: list(jexl.validate("##stuff\ntrue"))
Out[7]: ['Could not parse expression: ##stuff\ntrue']

In [8]: list(jexl.validate("//stuff\ntrue"))
Out[8]: ['Could not parse expression: //stuff\ntrue']

[is2ei]

Hi @jaredkerim
This issue is interesting for me.
Which style is required // or ## ?