Making icon inclusion optional

[ashleynolan]

Just looking at the icons, at the moment it seems like we have a very all or nothing approach when using this module. If you include @f-icons you will literally get all the icons that come with it and the classes that come with those icons.

Would be good to do something similar to the grid whereby the each set of icons are linked to a variable so that only the icons needed are pulled in.

Not sure whether this should be opt-in or opt-out (so either by default get everything and set variables to false to exclude, or by default get nothing and set to true to include). May be able to have a mix and have the default setup only import icons we think will be the most commonly used.

Either way would be good to implement this as the more icons that go into the repo, the worse this situation will get.