Add a command to install modules from requirements file

[stevenabadie]

It would be useful to track required libraries for a project with a library requirements file, like requirements.txt. Maybe this could be added to the existing install function with an -r option that takes a file name. This would install all of the libraries to the device. circup install -r requirements.txt

Additionally an option could be added to freeze to save the output to a requirements file. Otherwise, could just document to use the normal direct output to the file. circup freeze -r requirements.txt

[stevenabadie]

I've prototyped working changes to the freeze and install commands to now support requirements.txt. This is pretty bare bones as it is and could use further nuance but I think it does the trick for now to allow easier specification of libraries for projects. Any feedback before I add testing and make a pull request?