ignore whitespace separating value and inline comment

This adds support for having a `#` comment which is made inline, separated from a variable's value by whitespace.

Use case: I'm fond of quickly testing environment changes by modifying the value but keeping the original as a comment at the end of the same line. For example, my .env file might contain something like this:

    THIRD_PARTY_HOST=some.thirdparty.co.gov.edu.com

...but to get it to point to my own local version of that service, I'd modify it to look like this:

    THIRD_PARTY_HOST=localhost:4000 # some.thirdparty.co.gov.edu.com

When I tried this in a project using dotenv_elixir, I found that the above `THIRD_PARTY_HOST` would have this as the value: "localhost:4000 " — note the space after the last 0.

This commit makes the "unquoted value" matcher (in `Dotenv`'s `@pattern` regex) non-greedy, so that the `\s*` in the trailing "optional comment" section picks up any trailing whitespace.