Fix prompt issue for non-login subshells in OS X

This commit fixes an issue with themes on OS X which are sometimes
left in a partially broken environment with missing dependencies.

When a `BASH_IT_THEME` is active on OS X and has a
dynamic `PROMPT_COMMAND` which is not exported, and such
`PROMPT_COMMAND` is backed by shell functions which are not
exported either, and at the same time the theme is not
OS-X-aware (regarding `update_terminal_cwd`), and the
user launches a (non-login) interactive subshell from
the OS X Terminal, a `command not found` appears on every
command invocation.

The issue is caused by a regression in PR #514, which attempts to
inject `update_terminal_cwd` into the prompt. As a side effect, it
also escalates the exportedness of `PROMPT_COMMAND` while the
theme-specific backing functions (which power the dynamic prompt)
remain unexported.

The subshell cannot recover from this partially broken environment
because unlike in Linux, Bash-it on OS X is not invoked for non-login
subshells. The dependencies remain broken which leads to the error.

The fix is to preserve `PROMPT_COMMAND`’s exportedness on OS X,
leaving the individual theme responsible for consistently
exporting either all or nothing of its environment.

plugins/available/osx.plugin.bash

@@ -5,7 +5,10 @@ about-plugin 'osx-specific functions'
 if [ $(uname) = "Darwin" ]; then
   if type update_terminal_cwd > /dev/null 2>&1 ; then
     if ! [[ $PROMPT_COMMAND =~ (^|;)update_terminal_cwd($|;) ]] ; then
-      export PROMPT_COMMAND="$PROMPT_COMMAND;update_terminal_cwd"
+      PROMPT_COMMAND="$PROMPT_COMMAND;update_terminal_cwd"
+      declared="$(declare -p PROMPT_COMMAND)"
+      [[ "$declared" =~ \ -[aAilrtu]*x[aAilrtu]*\  ]] 2>/dev/null
+      [[ $? -eq 0 ]] && export PROMPT_COMMAND
     fi
   fi
 fi