eval: Added eval command

* Added a simple eval command which evaluates usper specified
  expression which can contain calls to functions from the `math`
  module.

* Added a sandboxed environment which should prevent abuse of this
  command.

* Added a bit longer docstring that could serve as an example of the
  formatting brutal plugins could use in the future.

Signed-off-by: mr.Shu <mr@shu.io>
Merge pull request #21 from mrshu/mrshu/eval

eval: Added eval command