Added a .gitignore for obj/ and bin/ directories.
New tests for the border-width setting are passing.
The new tests for the 'icons-icon' option passes.

This required me to make explicit the difference between an option which was
_allowed_ to be zero, and one which was not. To that end, there is now a
`try_parse_ulong_nonzero` to make the difference explicit.
Added some tests for non-numeric config values.

This required some changes to `try_parse_ulong` to figure out when an
input is not numeric, as well as the addition of the tests themselves.

Also, I forgot to commit `src/configparse.cpp` last time.