$XDG_CONFIG_HOME with ~ notation is ignored by tpm

[jaclu]

tpm does not do os.path.expanduser() on $XDG_CONFIG_HOME

Thus insisting on installing plugins to ~/.tmux/plugins unless that variable has been expressed with a full path.

The fix is trivial, and I can supply that PR, I just wanted to first check if this is by intent, or if It is just an oversight.