Make use of Facebook and Linkedin YAML variables.

[groundh0g]

See commits and comments in #241.

[groundh0g]

Edit _config.yaml to include:

email : blah@email.test
github : username
twitter : username
>> facebook : username
>> linkedin : username
feedburner : feedname

Unused ATM, but could be handy for new themes.

[groundh0g]

Bumping back to current milestone...

1. Add variables to _config.yaml in master.
2. Add reference to new vars in existing themes?

[groundh0g]

Added variables to config, but didn't update the existing themes. New themes can reference them moving forward.