add a dedicated file for functions (.functions.sh)

.bashrc is getting quite big, I've decided to split it and introduce
a new file just for functions (.functions.sh). It's called from the
.bashrc so no other action is needed.
add a dedicated file for prompt (.prompt.sh)

.bashrc is getting quite big, I've decided to split it and introduce
a new file just for prompt (.prompt.sh). It's called from the
.bashrc so no other action is needed.