Don't override customized environments in subshells

Only set up environment for the top-level shell, and allow it to be
inherited normally.

A top level shell is usually a login shell, but can also be a
non-interactive, non-login shell in certain cases, such as when
executing an SSH remote command.

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
Add Rails 4 bin path
Rewrite _rails-command to work from app subdirectories
Do not specify a bundler install path
Git ignore bundler vendor/assets
Comment a typeset
[Fix #362, Fix #384] Set Mac OS X 10.6 terminal window title
Remove unused GNU Screen window number format
Rewrite terminal auto-title
Simplify auto-titling
Prefix internal functions
Unsufix internal functions
[Fix #324] Add configurable terminal window and tab title formats
Remove utility aliases for Cygwin
Warn that Cygwin is not supported
[Fix #503] Prefix rails aliases with 'bundle exec'
Fix heading level for "zpreztorc"
Update copyright
Update external syntax-highlighting
Update external history-substring-search