Add possibility to blacklist some domains

[angrybacon]

The inserted button sometimes breaks the layout (keep.google.com for instance).

I see 2 options:

• With https://github.com/stsquad/emacs_chrome/issues/120: by altering ourselves the styling to comply with the visited websites
• By blacklisting related domains

Nonetheless, I think it is a useful feature to be able to blacklist domains.

Edit: By blacklisting domains, I mean that the buttons shouldn't appear on said domains.

[fstamour]

This is almost a duplicate of #107