Extension plugin support

[th-we]

@annplaksin and I worked on how to export custom symbols and text styles as sensible MEI, without having to fork Sibmei. For this, we reworked the entire text and symbol export. This brings quite some substantial changes to the code, and the following benefits:

• Better conversion of formatting for all texts
• text substitution variables work everywhere (see Text subsitution variables #152)
• large switches were replaced by declarative definitions of symbols
• fermatas were improved while porting them to the declarative approach (Fermatas over rests get exported twice #89)

We wrote documentation and an example.

[annplaksin]

Are there any objections about this PR? If not, I would like to merge it maybe next week or so...

[ahankinson]

I can merge it

[annplaksin]

Thanks a lot!

[ahankinson]

THank you (and @th-we ) for the work!