Feature: Adding file types filter & F flag changed

-t = Show summary of types

-e = Filter by regex
	allows you to specify a file type like -e "\.txt$"

Change behaviour of '-f' flag - it now counts only files. Before it
counted files & directories. This was needed for compatibility with
the new '-e' filter flag

README.md

@@ -57,6 +57,8 @@ Usage: dust -b (do not show percentages or draw ASCII bars)
 Usage: dust -i (do not show hidden files)
 Usage: dust -c (No colors [monochrome])
 Usage: dust -f (Count files instead of diskspace)
+Usage: dust -t Group by filetype
+Usage: dust -e regex Only include files matching this regex (eg dust -e "\.png$" would match png files)
 ```