Allow URL source for address book sections

[aspiers]

Once #16 is implemented to allow multiple sections to the address book, rather than requiring every section to be plain text pasted in, we should allow some sections to be retrieved via a URL.

This could allow retrieval from:

• a web server allowing shared access to a single section, which could even be public, e.g. https://raw.githubusercontent.com/...
• file:///... if the browser extension security model allows that.
• Rotki's private and global addressbooks (tracked separately in allow automatic importing of Rotki's private and/or global addressbooks #81)

This would open up many use cases and different models for storing and sharing address book data.

[LefterisJP]

For rotki integration we could probably use: https://rotki.readthedocs.io/en/latest/api.html#post--api-(version)-names-addressbook. It's filterable and you could probably get all names without any options. If not we can adjust.

[aspiers]

@LefterisJP Thanks, have filed #81 to specifically track the Rotki integration!