bak plugin: add unbak function (inverse to mvbak)

plugins/bak/unbak.fish

+# Function to move files and directories (a.txt.20140608_195859.bak move to a.txt)
+# (c) Roman Inflianskas (rominf) <infroma@gmail.com>, 2014
+
+function unbak
+	set re_bak '(.*)\.[0-9]{8,8}_[0-9]{6,6}\.bak'
+	set file $argv[1] 
+	set normalized (echo "$file" | sed -E "s/$re_bak/\1/g")
+
+	function is_bak
+		echo "$file" | perl -ne "print if /$re_bak/" > /dev/null
+	end
+
+	if test ! -e $file
+		echo "File \"$file\" not exists! Cannot unbak \"$file\"."
+	else if not is_bak $file
+		echo "File \"$file\" don't meet bak files convention! Cannot unbak \"$file\"."
+	else if test -e $normalized
+		echo "File \"$normalized\" exists! Cannot unbak \"$file\"."
+	else
+		mv $file $normalized
+	end
+end