Allow domains to be imported and exported

[dpacassi]

When people are switching from other web extensions, they might have a list of black- and/or whitelisted domains - we should implement a feature that allows us to import and export our data.

The export functionality would be great for backups or synchronizing until #8 is implemented.

[johnwmail]

+1 import and export domain list

[phemmer]

For those coming from Chrome's Quick Javascript Switcher, wanting to import their block list on this extension, and with the technical know-how to follow the instructions below:

• Export the contents of the Quick Javascript Switcher blacklist into a file.
• Using jq, convert the contents into this extension's format:

jq '[.[]|select(.setting=="block") | .primaryPattern | match(".*?//([^/]+).*").captures[0].string] | map({(.): now|todateiso8601})|add' < /path/to/file

• Go to your firefox profile directory and edit browser-extension-data/{41f9e51d-35e4-4b29-af66-422ff81c8b41}/storage.js. (unsure if this path varies, but this was the location on my system)
• Insert the output from jq into the object in the editor (adjusting braces & comma as appropriate).