Multiline op statemenets

- Improved `dot-exp` `bnf` grammar, fixes #44;
- Added customisable offset for the multi-line statements with hanging operator, as requested in #44;
- Renamed `swift-indent-multiline-dot-offset` to `swift-indent-multiline-statement-offset` to make it more generic.

I hope it's not too late to rename this parameter, I didn't account other multi-line cases initially.