Add a Command class; add Entry.getFillText()

getFillText() serves to decouple the text displayed in the auto-complete list
via getText() and the text actually filled into the box via getFillText(). This
allows us to display command + args on the list but only fill the command part.

A Command class has been added to provide some structure when extracting the
command name and args. Manually tested and it works.

src/components/structures/RoomView.js

@@ -417,7 +417,7 @@ module.exports = React.createClass({
         }
         this.tabComplete.setCompletionList(
             MemberEntry.fromMemberList(room.getJoinedMembers()).concat(
-                CommandEntry.fromStrings(SlashCommands.getCommandList())
+                CommandEntry.fromCommands(SlashCommands.getCommandList())
             )
         );
     },