Allow use of absolute paths in ignore-files

[outtersg]

This PR resolves #518: unable to use absolute paths in --ignore-files=

[outtersg]

This first iteration of this PR partly resolves #518 (on POSIX systems, so ironically not on @fatsen's system).

While it works for my use case this PR comes with 3 limitations:

1. it hardcodes / to detect an absolute path; so, only works on POSIX and such. No Windows here, sorry.
2. it does so by cloning Query::select()'s code
   And code duplication is bad.
3. It doesn't expand the usual ~ shortcut, so for example my .ugrep needs to contain a full:
   ignore-files=/home/gui/.ugrep.ignore
   while it would be nice to have a simple (and relocatable):
   ignore-files=~/.ugrep.ignore

For these reasons, I consider this PR a first step towards a more general solution that would include a mutualized and robust Path class, 1. able to work on Windows, 2. used by both ugrep.cpp, ugrep-indexer.cpp, and query.cpp, and 3. detecting at least the ~ special pattern (mapped to getenv("HOME") on POSIX, and I don't know what on Windows).

… But as "Perfect is the enemy of good", this PR has the merit of being there.

[outtersg]

Now this should correctly handle ~, as well as @fatsen's Windows absolute paths, removing 2.5 of the 3 points in my previous comment (there's still 0.5 of code duplication, as I didn't plug it into query.cpp).

This uses a new Path class, with a single static from_dir() to handle those paths normalizations.