Add ability to evaluate current and outer block

[abogoyavlensky]

The main feature that was added in this PR is:

• if the selection is empty try to find a nearest current block and evaluate it;
• evaluate selection if it exists;
• otherwise, if the current block couldn't be found eval whole file.

The same for an outer block.

Here block means everything between round brackets (...). It is slightly different than Atom's Proto Repl implementation which offers evaluation of vectors [...].

Caveats:

• doesn't handle any string literals, so unmatched round brackets inside string could brake block evaluation.

Resolves #61

A short demonstration: (keybindnigs used for recording: ctrl+n: "Eval and show the result", ctrl+alt+e: "Eval")

[abogoyavlensky]

And the UX of this feature could be still a matter for discussion. In PR I suggest this option:

• don't add any additional commands;
• use "Eval and show the result" for current block evaluation and "Eval" for outer block evaluation;

Cause in my opinion most likely we use plain compilation without result for function definition or other definitions with probably many other expressions after they have been changed somewhere inside. And we use current block evaluation when the result is more important for us. It slightly changes the default behavior so that to eval entire file you need to move the cursor outside of any blocks.