Forms Functionality: Be able to view forms and submissions

[lannonbr]

It was suggested in Issue #6 and I have added a new command and one subcommand to netlifyctl: $ netlifyctl form.

Running the forms command will list out every form a user has access to with the following fields: form name, form id, site id, and number of submissions.

As well, you can view basic details of form submissions as follows:

$ netlify form submissions --form-id <form-id string>

That will return a list of the submission timestamp, name, and body.

[calavera]

This is great @lannonbr! Thanks a lot.

[calavera]

I'm merging this, but I'm going to wait a few days to do a release, although the command should be available via go get before that.