Add shortcuts for the main hosts file to compare.

[StevenBlack]

Presently if we just say,

$ ghosts

We use default to https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts as the main hosts file.

Introduce shortcuts, such that

$ ghosts -m f  ## uses the fakenews hosts file

$ ghosts -m fs  ## uses the fakenews and social media hosts file

$ ghosts -m gfs  ## uses the gambling, fakenews, and social media hosts file

The shortcuts are to be in alphabetic order.

Use b or base as a shortcut for the base hosts file.

[StevenBlack]

Still todo:

• Document the shortcuts
• Tests for the shortcuts