Expand ~path file names for hoogle calling.

The current document suggests adding path elements like "~/.cabal/bin"
to the PATH environment variable and the `exec-path' variable.  This
works well enough when your default shell is Bash, but I had some
trouble with it when using zsh as my default shell.  In particular,
efforts to call hoogle locally would error out with the message:

    zsh:1: command not found: hoogle

A better approach is to wrap elements like this ("tilde paths") in a
call to `expand-file-name', which will expand them to absolute paths - a
situation which every shell I've tested can deal with properly.