Support required keywords in @mock

[omus]

Fixes #85

[codecov]

Codecov Report

Merging #86 (ad6bb8d) into master (dd9e74d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #86   +/-   ##
=======================================
  Coverage   83.19%   83.19%           
=======================================
  Files           6        6           
  Lines         119      119           
=======================================
  Hits           99       99           
  Misses         20       20           

Impacted Files	Coverage Δ
src/expr.jl	100.00% <100.00%> (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 dd9e74d...ad6bb8d. Read the comment docs.

[omus]

I'll merge in a couple hours in the off chance someone wants to review this