Implement configuration file

[quintesse]

Would be useful to have the option to set common options like the cache folder and default repositories to use in a user config file.

We could look for $HOME/.config/jpm/config.yml and $HOME/.jpmcfg.yml with the following contents:

config:
  cache: ~/jpmcache     # Sets the default value for the `--cache` command line option
  directory: libs       # Sets the default value for the `--directory` command line option
  no-links: true/false  # Sets the default value for the `--no-links` command line option
  repositories:         # Sets the default value for the `--repositories` command line option
    dummy: https://dummy