Add handlebar math operators

[moretea]

@IxDay opened the issue habitat-sh/core-plans#497, in which he describes two things that are needed to make zookeeper work.

One of them is that we need a "+" operator to construct the correct zookeeper id.

I believe that in general it would be useful to have several math operators, given that there is no other layer where logic can be run.

[smith]

There's a ton of JavaScript helpers here that probably could be copied to the rust versions fairly easily.

I know I would like an equality helper, to compare two numbers as well.

We should be circumspect in how many of these we add though, as putting too many in at once might just make things harder.

[moretea]

See also #2223. Maybe we should open a tracking issue? There is a list of helpers in https://github.com/helpers/handlebars-helpers

[reset]

@moretea I think you wanted to link to another issue, that's the number of this one :). Did you mean #2281 by chance?

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