Add m textobject for pair under cursor

[Omnikar]

Closes #956.

[sudormrfbin]

This also needs to work with the nearest surround pair even if the cursor is not exactly on a surround character. For example if I press mim on any character between the braces in this text

{sentance}

it should automatically detect the braces as the nearest surround pair and select the text inside it.

[Omnikar]

Hm, that's going to be tougher to implement.

[EpocSquadron]

Hm, that's going to be tougher to implement.

I think we should leave it at the current functionality. There's a bit of precedent, in that pressing mm only works if you are on a character. I don't disagree that it would be cool for it to pick the first braces around the cursor though, so we should open a new issue to explore how that should work.