Make it possible to point to a different maskfile

[felipesere]

This PR adds the ability to specify --maskfile or -m followed by a path to a maskfile.

I had to work around the fact that clap will still parse all of the arguments from argv, so I had to add a subcommand to match --maskfile (see the comment below).

Other than that I added a the dev-dependencies to start writing integration tests for the binary.

Let me know what you think!

[felipesere]

Solves #7

[jacobdeichert]

Hey Felipe, thanks for doing this!

I've got a busy weekend ahead, but i'll try to give this another look tonight or late tomorrow. Looks good at first glance 👍

[jacobdeichert]

Thanks again for doing this! It works great 💯.

This actually unleashes a new use case: global commands that people can use a bash alias to execute for simplicity. I'll document this after the merge.

Can you accept/commit the suggestions and then I'll merge?

[felipesere]

All changes should be up to date now.

[jacobdeichert]

Thanks for being so quick! 😄