Increase control over multiplexer auto-start

Setting the auto-start 'remote' and 'local' zstyles for the screen and
tmux modules to 'true' automatically starts a session in SSH connections
and in local terminals respectively.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Move from Carton to Cask

Carton was renamed to Cask to avoid a name clash with a Perl utility of
that name.
Clarify customization instructions

Explicitly state that a new user must create his own fork to commit and
track changes via Github.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Update external completions
Update external history-substring-search
Update external syntax-highlighting
Replace pythonz with pyenv

Depending on Python to manage Python is silly.
Add utility aliases for Cygwin

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Remove duplicate alias definitions
Simplify a conditional
Do not define $GEM_HOME

Defining it breaks gems for all users who neither use RVM nor rbenv,
but have a different default Ruby version nonetheless.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Use correct option for pacman search aliases
Stop module loading when external code fails to source
Comment an unnecessary style
[Fix #477] Add instructions for updating Prezto
[Fix #479] Start only one ssh-agent instance
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  [Fix #479] Start only one ssh-agent instance
  [Fix #477] Add instructions for updating Prezto
  Comment an unnecessary style
  Stop module loading when external code fails to source
  Use correct option for pacman search aliases
  Do not define $GEM_HOME
  Simplify a conditional
  Remove duplicate alias definitions
  Add utility aliases for Cygwin
  Replace pythonz with pyenv
  Update external syntax-highlighting
  Update external history-substring-search
  Update external completions
  Clarify customization instructions
  Move from Carton to Cask
  Increase control over multiplexer auto-start