Parse lines beginning with "export"

[atomaka]

We commonly attempt to reuse our .env files by including export at the beginning of each line. This allows our .env files to also be sourced and loaded into our environment if necessary. Similar dotenv packages seem to allow this functionality (ie. https://github.com/bkeepers/dotenv#usage).

This pull request maintains all existing functionality. It adds one additional change. A line formatted as

export EXPORTED=true

will be parsed with key EXPORTED and value true.

[coveralls]

Coverage remained the same at 100.0% when pulling 14d6beb on atomaka:feature/allow-export into 825c1b2 on motdotla:master.

[atomaka]

Looks like this functionality was proposed and declined in #118 (although the suggested solution was pretty disappointing).