Add custom sources

Create a way to add custom sources for learning materials i.e. personal git repos. A possibility would be to make .rwth.toml customizable. Such a setup could look like this:

.rwth.toml:

(...)

[custom-source]

[custom-source.my-module-ss18]
name = "my-module"
url = "gitlab.com/me/my-module-ss18"
semester = "ss18"
folder = "my-module"
command = "git pull"

Which would pull updates from the specified repository when performing $ rwth_cli sync ss18 my-module.