Add `around quote` text object, exclude whitespace in `a quote` text object

This makes quote text objects more consistent with the other text objects, regarding white space handling. Initially `da"` would include whitespace in one direction do be compatible with Vim's stock text objects. Unfortunately that made it impossible to select a quote without surrounding white space. This pull request fixes that.

New quote text objects:
```
c ' dddd ' e
  ││└I'┘│││
  │└─i'─┘││
  ├──a'──┘│
  └──A'───┘
```



Close #63.