add basic "set" command, with "ft" and "tw" options

lib/ex.coffee

@@ -165,6 +165,28 @@ class Ex
     else
       pane.splitUp(copyActiveItem: true)
 
+  set: (args...) ->
+    editor = ->
+        atom.workspace.getActiveTextEditor()
+
+    arg   = args[1]
+    regex = /\s*(\w+)(?:\s*=\s*([\d\w]+)\s*|(!))/
+    matches = arg.match regex
+    return unless matches?
+
+    [whole, option, value, bang] = matches
+
+    switch option
+      when 'tw'
+        editor().setTabLength(parseInt(value))
+      when 'ft'
+        regex = new RegExp "(source|text)\.#{value}$"
+        lowerCaseName = value.toLowerCase()
+        for name, grammar of atom.grammars.grammarsByScopeName
+          grammarName = grammar.name.toLowerCase()
+          if name.match(regex) or lowerCaseName is grammarName
+            editor().setGrammar(grammar)
+
   sp: (args...) => @split(args...)
 
   substitute: (range, args) ->