Allow exclude/ignore by absolute path

[cyqsimon]

Closes #851.

This is a crude implementation of my idea in #851 (comment). I don't think it's ideal (far from it), but at least we have some kind of support for people who want it.

I'll keep it a draft for now. Please chip in your ideas on how this could be improved.

Main sore spots

• Performance penalty due to path canonicalization (although this is unavoidable)
• Globs work somewhat differently due to implementation
  • Excluding a directory doesn't cause its children to be excluded too
    • To exclude /foo/bar and all its children, it's necessary to specify a glob of /foo/bar/*
• The introduction of a separate flag and global ignore file for this purpose
  • --exclude-absolute and ~/.config/fd/ignore-absolute

Todo list

• Implementation
• Tests
• Documentation