Use maplike<K, V> instead of custome methods / attrs.

[yutakahirano]

WebIDL now supports maplike member and WebMIDI should use it. #115
As ReSpec has not support it yet, this change uses a custom script as a
workaround. This should be fixed when ReSpec supports maplike.

[yutakahirano]

The script runs as follows.

• If the browser doesn't support MutationObserver, do nothing (i.e. maplike<...> doesn't appear in the idl). The descriptions "... as a maplike interface readonly maplike<DOMString, MIDIInput> whose key is ..." remain valid.
• Otherwise, insert "readonly maplike<DOMString, MIDI[Input|Output]>;" into the IDL when ReSpec creates it.