If the Ruby on Rails grammar depends on HTML, but it isn't loaded, its
syntax highlighting won't include HTMl tokens. If we later load HTML,
we should update any buffer with the Rails grammar to reflect the
change. This commit changes grammars to memoize their initial rule and
repository. If an included grammar is added or removed, we clear the
memoized rules and emit a 'grammar-updated' event. Any tokenized
buffer that points to this grammar can then retokenize to reflect the
newly available/unavailable included grammar.
