strip export statement to be compatible with other dotenv implementation

[florisvink]

Dotenv autoloaders like oh-my-zsh needs the explicit shell export statement at the beginning of the line. This PR handles these .env files as well.

[coveralls]

Coverage remained the same at 100.0% when pulling 7e2b2d3 on OOShoppingnl:master into ad86907 on motdotla:master.

[maxbeatty]

😕 the dotenv plugin you linked to says "export is optional"

This has been proposed a number of times and the thinking is still to support a consistent INI file format and encourage other modules that build on top of it from there

[dosentmatter]

If anyone wants to allow exporting all variables in the .env file, you can do the following:

set -a # Turn on allexport
. ./.env # source .env
set +a # Turn off allexport

set -a is POSIX compliant so it should work with most shells. Then you just don't include the optional export for all dotenv implementations.