Add RegEx support for ignoring files

[BrandonStaab]

Currently a file/directory can be ignored with --ignore-glob <pattern>. RegEx can express things that Unix globs can not and are more familiar to some people. A possible use case for adding this feature would be ignoring the contents of a .git or build directory while still showing that the directory exists when using the --tree flag. It is no immediately obvious that this can be done using Unix globs and somebody else may have a use case that can not be represented using them.