Added Text.Pandoc.Asciify utility module.

This has functions to create ascii-only versions of identifiers.
Options: Added `Ext_ascii_identifiers`.

This will force `Ext_auto_identifiers` to use ASCII only.
Set as default for github markdown.

Note: API change.
Markdown reader:  Implemented `Ext_ascii_identifiers`.

Closes #807.
README: Documented `ascii_identifiers` extension
README: Documented markdown variants (which extensions they have).