maybe overkill but would be nice if this implemented the actual tilde expansion spec[1] - could probably use a very straight port of Python's `expanduser` [2]

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_01
[2] http://hg.python.org/cpython/file/c5aa95c7b5e5/Lib/posixpath.py#l258