Support package trees containing absolute symlinks

[aspiers]

[This feature request was originally confused with #11 and reported there; moving here to stop the confusion.]

Example use case given by @dmarcoux is a Stow package containing dotfiles for $HOME, in which .mysql_history, .bash_history, and .less_history are symlinks to /dev/null.

Currently Stow refuses to stow these absolute symlinks:

WARNING! stowing home would cause conflicts:
  * source is an absolute symlink dotfiles/home/.bash_history => /dev/null
  * source is an absolute symlink dotfiles/home/.lesshst => /dev/null
  * source is an absolute symlink dotfiles/home/.mysql_history => /dev/null
All operations aborted.

To clarify:

• This issue is about package trees containing absolute symlinks.
• Should we add support stow directory is symbolic link. #11 is about the Stow directory being a symlink.
• Create symlinks with absolute path names #3 is about the symlinks which Stow manages in the target directory being absolute symlinks.

They're all separate issues despite appearing similar on the surface.