Implement save command strategy gdb

@danschumann originally came up with this strategy in #44

CHANGELOG.md

@@ -4,6 +4,7 @@
 - plugin now uses strategies when fetching pane full command. Implemented
   'default' strategy.
 - save command strategy: 'pgrep'. It's here only if fallback is needed.
+- save command strategy: 'gdb'
 
 ### v1.3.0, 2014-09-20
 - remove dependency on `pgrep` command. Use `ps` for fetching process names.

save_command_strategies/gdb.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+CURRENT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+
+PANE_PID="$1"
+
+exit_safely_if_empty_ppid() {
+	if [ -z "$PANE_PID" ]; then
+		exit 0
+	fi
+}
+
+full_command() {
+	gdb -batch --eval "attach $PANE_PID" --eval "call write_history(\"/tmp/bash_history-${PANE_PID}.txt\")" --eval 'detach' --eval 'q' >/dev/null 2>&1
+	\tail -1 "/tmp/bash_history-${PANE_PID}.txt"
+}
+
+main() {
+	exit_safely_if_empty_ppid
+	full_command
+}
+main