unicode_to_latex.py: improve output correctness

I encountered a few mappings that only worked in math mode, not text mode.

Also we now normalize the input Unicode; otherwise combining characters
would come out wrong.
bibtexparser: import a copy of version 0.5.1

For simplicity I'm just duplicating the code. Ideally it'd be a Git submodule
or a dependent module, but the former is just very confusing for most people,
and the latter makes people do too much work.
wltool: implement bibtex bootstrapping

It's kind of awkward, but gets the job done for my test case.
wltool: actually make subcommand help work; supply it for bootstrap-bibtex
README.md: document bootstrap-bibtex