Add new text object commands i", a", i( and a(

[fukamachi]

Text objects i" and a" are commands to handle a string object quoted by ".

Lem provides maybe-beginning-of-string, however, I implemented them by myself.
Because Vim doesn't depend on syntax table to do it, and it would be useful for other quoted text object like i' and a'.

[cxxxr]

Thank you!