Fix parser regressions from multiline support

Version 1.5 came with a whole new parser, and with a new parser comes new bugs.

Things that were broken in 1.5 that are now fixed:

• unquoted variables with interior whitespace no longer split on the first space (and then break the following line if you have one)
• inline comments now work again for both quoted and unquoted variables
• export statement filtering was made more robust and matched earlier versions behaviour
• FOO.BAR key names are permitted again (i have no idea why you'd do it, but it's explicitly supported in ruby dotenv files)

There's one breaking change: earlier versions of this library would allow unterminated quoted variables in some instances and return a value (ie FOO="bar would set env of FOO: '"bar'), this now returns an error.

What's Changed

• Fix bug where internal unquoted whitespace truncates values by @joho in #205

Full Changelog: v1.5.0...v1.5.1