Optional auto-switching

[estum]

This commit provides the same workflow as chruby's auto.sh. When auto.sh script loaded into environment, it makes gem_home to automatically use a .ruby-gemhome to get project's $GEM_HOME path (.ruby-gemhome filename is customizable by the $GEM_HOME_FILENAME variable).

[postmodern]

One major problem with this is that you cannot have multiple traps on bash. It's also very difficult to get the current trap command and append to it.

[calebhearth]

@postmodern Would it be possible to detect the chruby auto switching here and set up a trap that does both things? Likewise in chruby's auto we could detect the gem_home auto behavior and set up the trap.

if bash
  if $USING_CHRUBY_AUTO
    trap gem_home and chruby stuff
  else
    trap gem_home stuff
  end
end