Add a check for the pasteParsedInMathMode

The default functionality of paste wraps any string without enclosing
$'s in a latex text command. If `pasteParsedInMathMode` is true any
$-unwapped string will be parsed in the in Math Mode.
Add test cases for pasteParsedInMathMode
Change pasteParsedInMathMode to statelessClipboard option

As discussed in #269. Not documented in README yet because it doesn't
really work, the whole point is consistency between math and text mode,
but all that's implemented is the subset that makes copying and pasting
math in math fields work.
Merge pull request #269 from mathquill/fix.pasteOpts

Add statelessClipboard option