Fix webfont application condition

When queueing elements for applying webfonts, elements are
supposed to be queued if a relevant font family was found.
If it's not found, it's undefined. The condition checked for null,
so some elements that didn't need webfonts received them.
The condition is fixed in this commit.

This fixes
https://bugzilla.wikimedia.org/show_bug.cgi?id=62530
Merge pull request #41 from amire80/nonull

Fix webfont application condition
Introduce overridable font family option

Font stacks given in overridableFontFamilies will be overridden
as as exception to the general policy of not altering elements with
explicit font family styles

See https://bugzilla.wikimedia.org/63718
Update README.md
Merge pull request #42 from santhoshtr/overridable

Introduce overridable font family option