Add parser to env and argv values.

Introduces an optional `parser` parameter to the `.env` and `.argv`
config loaders, which is used to parse incoming string values to
JavaScript literals.  If the parser throws an exception, the original
string value will be used.  The parser defaults to JSON.parse.
Merge pull request #11 from srijs/feature/parse-argv-and-env

Add parser to env and argv values.