Update command substitution notation

Just a quick update to replace all instances of the older `` command substitution, to newer $() notation.
Add option to show extended git status information.

Add an option 'BULLETTRAIN_GIT_STATUS' to allow user to show extended git repo status; e.g. added, modified, deleted files instead of just clean/dirty.
Add an option to show extended directory/path information.

Add a user selectable option 'BULLETTRAIN_DIR_EXTENDED' to show extended directory/path information. Currently this is fixed at truncating the path shown at 3 folders deep, but could be modified to support user selectable nesting/truncation.
Add an option to show extended exit code/status information.

Add a user selectable option 'BULLETTRAIN_EXIT_SHOW' to show the actual exit status code ($RETVAL) along with the graphical indicator.