Add requirement.txt support for freeze and install commands

[stevenabadie]

This adds support for managing project module requirements through a requirements.txt file. The freeze command can now output a requirements.txt file for all modules installed on a connected device. The install command can then use the requirements.txt file to install a list of requirements to a connected device.

The expected workflow would be that a project could export a requirements.txt file with the modules required for their project and provide the file in the project source. Another user could then quickly install all of the required modules for the project onto their device.

This is built on top of the existing install command which does not consider module versions. For later I think as there would need to be a way to organize all of the module versions.

Close #29

[FoamyGuy]

I tested circup freeze -r and circup install -r requirements.txt on Win 7. Both are working as intended for me. Really like this functionality.

[ntoll]

Hey @stevenabadie, you're on fire 🔥 👍

Thank you, once again, for this contribution. Please remember to make check before submitting a PR since it runs the full test suite and reports on code coverage. I've added a # pragma: no cover to the install_module function since we're not unit testing those aspects of the code which require IO interaction with the user (everything else, which does the heavy lifting, is tested).

I also took the liberty to check on the pip file format guidelines (https://pip.pypa.io/en/latest/reference/pip_install/#requirements-file-format) and made two small, but hopefully useful, changes to your code:

1. Empty lines are ignored (they were reporting an error).
2. Comments are honoured (Anything after a # is ignored).

As a result, a requirements.txt like the following will work:

adafruit_rsa==1.1.0
adafruit_seesaw==1.6.0

adafruit_tinylora==2.1.0  # this is a test.
# Another comment.
adafruit_waveform==1.3.0

Other than that, it's a beauty and delighted to merge..! 🙂

[stevenabadie]

@ntoll thank you for reviewing, feedback, and additions! 🙌