Add function to expand ~ in .stowrc files (#14)

Add a new expand_tilde() function that performs tilde expansion of
strings, and corresponding unit tests:

    * A ~ at the beginning of a path is expanded to the user's home
      directory.
    * Literal '~' can be provided with '\~'

Combine this with expand_environment() in a new expand_filepath()
function which applies all (both) required expansion functions to a
string, and use that in get_config_file_options() to expand .stowrc
options.

Add more tests to check that tilde expanded in correct places, i.e.:

    * expanded for --target and --dir
    * not expanded for --ignore, --defer, or --override

Update documentation on stowrc files according to this functionality
change.

Fixes #14: https://github.com/aspiers/stow/issues/14

NEWS

@@ -22,6 +22,16 @@ News file for Stow.
 
     Thanks to Joris Vankerschaver for this feature!
 
+*** Shell-like expansion in .stowrc files
+
+    For options within .stowrc files which describe file paths, "~" can
+    be included to expand to the current value of $HOME, and
+    environment variables can be referenced e.g. via "$FOO" or
+    "${FOO}".  To prevent expansion, escape with a backslash.
+
+    Thanks a lot to Charles LeDoux for his diligent work on this
+    feature!
+
 *** chkstow now honours the $STOW_DIR environment variable
 
     The stow script already honoured the $STOW_DIR environment