[subset] Handle non-existing ScriptList

As seen in CinzelDecorative-Regular.otf.
Convert warning to warnings module
Use True/False instead of 1/0
[subset] Add allowVID option to load_font()

Not enabled right now.
Handle out-of-range glyphIDs in ClassDef
Simplify Coverage table postRead
[subset] Minor
Minor
Minor
Fix SingleSubstFormat1 modulo math
Allow converting 'glyphXXX' to glyphID

There was an inconsistency: when allowVID was False, we were allowing
gid->glyphXXX, but not the reverse.  This was allowing some fonts to
be loaded, but then not to be written back.  Fix that.
Relax some warnings
[subset] Minor
[subset] Ignore out-of-range glyphIDs

We don't want to support out-of-range glyphIDs in cmap or GSUB.
Explicitly drop them.
Add symlink from Tools/fontTools to Lib/fontTools

This way, running tools inside Tools/ uninstalled will run against
the source tree version of the library.