Extract shared prompt values and refactor bash/git prompts

Move color constants and PS1 escape sequences from bash-prompt.sh into
a new shared rc/03-values.sh, and extract the user@host:dir fragment as
LWS_PS1_USER_HOST to eliminate duplication between bash-prompt.sh and
git-prompt.sh. Wire up git-prompt to use bash-git-prompt with the shared
prompt variables.

rc/03-values.sh

@@ -0,0 +1,12 @@
+# Terminal colors
+LWS_RED=$(tput setaf 1)
+LWS_GREEN=$(tput setaf 2)
+LWS_CYAN=$(tput setaf 6)
+LWS_COLOR_RESET=$(tput sgr0)
+
+# Bash prompt escape sequences
+LWS_PS1_USER='\u'
+LWS_PS1_HOST='\h'
+LWS_PS1_DIR='\w'
+LWS_PS1_TYPE='\$'
+LWS_PS1_USER_HOST="${LWS_GREEN}${LWS_PS1_USER}@${LWS_PS1_HOST}${LWS_COLOR_RESET}:${LWS_CYAN}${LWS_PS1_DIR}${LWS_COLOR_RESET}"