Add a callback to parse().

[ericsnowcurrently]

There are a couple of use cases in which dotenv.parse() doesn't quite meet my needs:

• when I need to see the value from each valid line; sometimes an env var is set multiple times but currently I only get the last entry (I need them all)
• when the the order from the .env file matters

This change addresses those by adding a callback to parse() which gets called with the key/value pair for each valid env var line. This could also be done with an iterator that gives the key/value pairs (and parse() would use that), but a callback seemed simpler.

[maxbeatty]

This use case is pretty far from the intended purpose of parse. Might be better off copying our code for your specific needs.

[ericsnowcurrently]

Fair enough. I'd already adapted your code locally but wanted to play nice. :)