Handle specialized template functions

[hidmic]

Precisely what the title says. It does so generalizing the regular expression used for function signature parsing in the doxygenfunction directive implementation.

[jakobandersen]

Looks interesting. Do you have some examples of what this enables?

[hidmic]

@jakobandersen sorry for the delay. See 17c29de.

BTW flake8 isn't happy with master as-is.

[hidmic]

@jakobandersen ping.

[jakobandersen]

Thanks!
Merged via rebase and a few extra commits (see aec4702 and its ancestors).
This regex indeed seems to fix the issue of function template specializations.

Notes for future improvements:

• It seems that the Doxygen lookup needs specific white-spacing of template argument lists, e..g, f< int > works, but f<int> doesn't. This requirement currently is exposed on the user side.
• The regex is getting complicated. When the lookup is changed, perhaps the parsing could be done with the Sphinx C++ domain instead. Though, we are getting into the icky problem that at this point Breathe technically doesn't know which language the lookup is for (this was mentioned/discussed somewhere I can't immediately find right know).