Fix ligature substitutions with space

White space fixes

Fix backtrack sequence for contextual ligatures
Generate contextual substitutions compatible with the currents version of
Harfbuzz.

Sort coverage table for ligatures

Merge branch 'master' of https://github.com/johanmattssonm/birdfont into HEAD

Write the context for contextual substitutions (ligatures)

Use unicode sequences in ligature tab
Allow the font designer to enter values on the form U+XX where
XX is the unicode value in hex.

Add abstract table class

Write and parse ligature subsitutions in GPOS table