Create lib finalize hook

Create an array `_bash_it_library_finalize_hook` and loop at the end of the main `bash_it.sh` to run each element in the array.

The purpose here is to run some command after everything else has been loaded. For example, the appearance lib checks for executables for SCM commands, but `$PATH` may be altered after appearance has loaded and therefore some available commands may never be discovered. Therefore, create `_bash_it_appearance_scm_init()` and add it to the hook. It will re-check at end of `bash_it.sh` just before prompt is first displayed.

themes/base.theme.bash

@@ -106,7 +106,7 @@ function _bash_it_appearance_scm_init() {
 		fi
 	fi
 }
-_bash_it_appearance_scm_init
+_bash_it_library_finalize_hook+=('_bash_it_appearance_scm_init')
 
 function scm {
 	if [[ "$SCM_CHECK" = false ]]; then