Add where syntax to rules

[rikhuijzer]

Closes #322.

@shashi, I think that it would be good to also test a more complicated rule with the predicate, but I couldn't get it to work. Could you either add it to the PR or give a suggestion?

[codecov-commenter]

Codecov Report

Merging #329 (78e5e2c) into master (aee8482) will increase coverage by 0.10%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
+ Coverage   83.89%   84.00%   +0.10%     
==========================================
  Files          12       12              
  Lines        1217     1225       +8     
==========================================
+ Hits         1021     1029       +8     
  Misses        196      196              

Impacted Files	Coverage Δ
src/rule.jl	86.77% <100.00%> (+0.93%)	⬆️

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[rikhuijzer]

Hey xD I'm still processing the very nice tip to use :($x ? $y : nothing) instead of Meta.parse

[shashi]

Oh no, I thought it's only in the tests. Sorry, would you mind opening another PR?

[rikhuijzer]

Oh no, I thought it's only in the tests. Sorry, would you mind opening another PR?

Np. Will be there in a few mins.