Implement :echo command and enhance expression evaluation

Fixes #641 

Implements `:echo` command:
+ Echoes any expression which is currently parsed by VsVim
+ Does not support multiple expressions the way vanilla Vim does (`echo 1 2 3`)

Enhances expression evaluation:
+ Evaluate register expressions (`@a`)
+ Evaluate option expressions (`&et`)
+ Evaluate variable expressions (`x`)

This does not:
+ Plumb the new expressions support through the whole system; e.g. `let x=@a` still does not work (though it should be an easy next step; perhaps I will submit another pull request for this---it would give me a sense of completeness)
+ Implement list or dictionary expressions (`echo {}` gives parse error)

Open questions:
+ Should I migrate some of the unit tests from `EchoTest` to `ExpressionInterpreterTest`?
Done. Sorry about that.