Add support for compact ActivityIndicator format

[deathaxe]

Resolves #175

This PR replaces ActivityIndicator.width attribute by ActivityIndicator.frames, to enable users to specify any sort of animation.

By default a compact single-char frameset is specified, but frames can also take a list of strings.

Example:

from sublime_lib import ActivityIndicator as AI

ai = AI(sublime.active_window().active_view())
ai.frames = ["[=   ]", "[ =  ]", "[  = ]", "[   =]"]
ai.start()