add math command

[jeremyandrews]

Add a command for performing basic mathematical calculation.

For example:

• What is (ten) (plus | minus | times | divided-by) (five)

[jeremyandrews]

The Snips number entity should make it fairly easy to create this command:

• https://snips-nlu.readthedocs.io/en/latest/builtin_entities.html#number

[jeremyandrews]

This is now working:

$ curl --data @test/ten-plus-ten.base64 http://127.0.0.1:8088/convert/audio/text
{"command":"simpleCalculation","human":"10 plus 10 equals 20","raw":"what is ten plus ten","result":20.0}