Updated #377 for merge

[thomasfedb]

Brought #377 in line with master, original PR below:

Adding support for creating templates from .env files

Templates can be checked in and will show the environment variables used but wont show the values

I love using this package while writing demos at Stripe. However every now and then I miss a variable in my README.md or a handcrafted .env.template.

This adds a flag to the cli so that you can point it at any .env file and safely create a template from it that doesn't include any of your keys. :)

[thomasfedb]

@cadwallion as requested 👍

[cadwallion]

Thanks to both @thomasfedb and @mikeshaw-stripe for this addition!