Add SPHINX_AUTOBUILD_DEBUG to aid debugging

[cmarqu]

Currently, changed paths and the ignore expressions are shown.

Closes #188

[AA-Turner]

Could we add a small test for this? Otherwise looks good.

[cmarqu]

I added a test.

This highlighted that currently, also SPHINX_AUTOBUILD_DEBUG=0 is considered to mean "print debug output" which might be confusing for users. WDYT?

[AA-Turner]

I agree, the empty string and 0 should probably be falsy.

A

[cmarqu]

I did this change, but to be really nitpicky, I'd consider the empty string truthy, and extend falsiness to the strings "no" and "false" (case insensitive).

It's surprising that there doesn't seem to be a commonly agreed upon best practice here.

[AA-Turner]

Thanks @cmarqu!