Adding a lexer to registry replaces any with the same name

Add HTML lexer and minor fix to Python lexer

Fix lexer errors and be less verbose

Fix infinite loop on Error token

Support the <combined> element

Some lexers (such as the Python lexer) use the <combined> element.

Fix README typo

Fixes for a few bugs

These were found when trying to use the Batchfile lexer:

* Support lexer usingself elements directly under a rule (instead of
being inside a bygroups)
* Support rules that include other rules that themselves include other
rules recursively
* More debug messages

Add markdown lexer

Add generated file

Update readme