Tai Tham and USE

This issue follows on from issue #170 with a proposal based on research into a visual encoding. In order to test such a proposal, I am proposing we make the following special cases for the USE for Tai Tham:

code | name |old category | new category
-------|---------|-----------------|------------------
1A56 | medial la | MBlw | SUB
1A5A | sign low pa | FAbv | MAbv
1A5B | high ratha or low pa | SUB | CMBlw
1A61 | sign a | VPst | GB
1A63 | sign aa | VPst | GB
1A64 | sign tall aa | VPst | GB
1A6D | sign oy | VPst | VBlw
1A74 | mai kang | VMAbv | VAbv
1A7A | ra haam | VAbv | FAbv
1A7B | mai saam | VMAbv | FM
1A7C | khuen-lue karan | FM | FAbv
1A7F | cryptogrammic dot | FBlw | CMBlw

Based on these changes, people can start implementing test fonts and keyboards towards deciding whether a visual encoding is the best solution for Tai Tham. In talking with folks interested in Tai Tham implementation, they all agree that this is the best first step forward. After all, Tai Tham is so broken in OpenType that having something is a good first step so long as people understand that this too **may change**.