Allow relative paths for options

[clphillips]

If a relative path is provided for any path option, resolve the path with respect to the --dir option or dir config file option.

For example:

vendor/bin/git-migrate --dir /full/path/to/repos --authors ../authors.txt

The --authors path would results to /full/path/to/authors.txt because /full/path/to/repos/../authors.txt resolves to /full/path/to/authors.txt.

This applies for the following options:

• --authors
• --items
• --javalib