Enhancement: Support negative include/exclude patterns

If a pattern starts with an exclamation mark and it matches a file that was
previously matched by a regular pattern, the match is cancelled. Notably,
this can be used with `--exclude-file` to cancel the exclusion of some files.

It works similarly to `.gitignore`, with the same limitation; Once a directory
is excluded, it is not possible to include files inside the directory.

Example of use as an exclude pattern for the `backup` command:

    $HOME/**/*
    !$HOME/Documents
    !$HOME/code
    !$HOME/.emacs.d
    !$HOME/games
    # [...]
    node_modules
    *~
    *.o
    *.lo
    *.pyc
    # [...]
    $HOME/code/linux/*
    !$HOME/code/linux/.git
    # [...]

https://github.com/restic/restic/issues/233
https://github.com/restic/restic/pull/2311
