add flag to disable warnings

[jrevels]

First of all, thanks for this package - it works great for auto-CSEing dual number primitive definitions in ForwardDiff.

After JuliaDiff/DiffRules.jl#11, some of these primitive definitions contain if statements, which basically means that whenever somebody loads ForwardDiff for this first time, they see a bunch of:

WARNING: CommonSubexpressions can't yet handle expressions of this form: if

For ForwardDiff's case, I don't mind if CSE can't be applied to an expression - I'm fine with this package just trying its best and handing me whatever the result is. Thus, this PR will allow me to disable this warning so users don't get scared.

If you could get a new tag quickly after this is merged, I'd be super appreciative. If that's too complicated, right now, though, I can hack around this warning in ForwardDiff in the meantime.

[rdeits]

Thanks, and I'm happy to tag a release as soon as this is merged. Can you just please check the test failure on Travis? It looks like a real error. I can also look into it, but I'm on mobile so it wouldn't be until tonight.

[jrevels]

Ah, forgot to add the warn_enabled argument to cse. Just pushed a fix, let's see if it passes now.

[codecov-io]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is 90.9%.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   97.67%   97.67%           
=======================================
  Files           1        1           
  Lines          43       43           
=======================================
  Hits           42       42           
  Misses          1        1

Impacted Files	Coverage Δ
src/CommonSubexpressions.jl	97.67% <90.9%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 912260d...ab2005f. Read the comment docs.