Index class needed

[jckantor]

Indexing is turning out to be one of the most useful features of nbpages. So far there are tag, python module, figure, and data indexes. Additional indexes that might be useful for notebook development are

• index of python error messages
• index of bibliographic references
• index resulting from a user search

Currently some of the indexes are cached, which makes it problematic to do rewriting for 'under-construction' tags. (issue #28).

Propose a new index class created with

tag_index = Index() tag_index.index(pattern) tag_index.write_html(title)

[adowling2]

Also include automated testing for the index class.