Update watch command with file monitoring

[lawmurray]

The doxide watch command implemented in #66 (see also #59) uses polling, i.e. it checks source files for changes on a regular interval. This was a deliberate choice as a first version, but it would be good to have a version that uses file monitors to detect when source files are changed, then update documentation, to reduce resource use.

Some candidate libraries that could enable this are:

• libfswatch
• Sane C++ Libraries
• GIO