Add bash prompt script for sourcing in bashrc

This only displays the left part of the prompt. It requires the
left-only theme to be able to display all segments.

The script must be sourced with an absolute path for now:

    . /path/to/powerline/bindings/bash/prompt.sh

Closes #90.

powerline/bindings/bash/prompt.sh

@@ -0,0 +1,23 @@
+_powerline_prompt_command() {
+	export PS1="$(powerline-prompt --last_exit_code=$? left)"
+	_powerline_tmux_set_pwd
+}
+
+_powerline_tmux_setenv() {
+	if [[ -n "$TMUX" ]]; then
+		tmux setenv TMUX_"$1"_$(tmux display -p "#D" | tr -d %) "$2"
+	fi
+}
+
+_powerline_tmux_set_pwd() {
+	_powerline_tmux_setenv PWD "$PWD"
+}
+
+_powerline_tmux_set_columns() {
+	_powerline_tmux_setenv COLUMNS "$COLUMNS"
+}
+
+trap "_powerline_tmux_set_columns" SIGWINCH
+kill -SIGWINCH "$$"
+
+export PROMPT_COMMAND="_powerline_prompt_command"