Fix and document @vars renaming with pair syntax

[knuesel]

This fixes the support for variable=>name syntax in @vars, and adds a few mentions of this syntax in the documentation.

There's also a minor fix in the Readme for the CI badge.

[jverzani]

Thanks! Will merge once all goes through.

[codecov-io]

Codecov Report

Merging #393 (213c0aa) into master (c929d0e) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #393      +/-   ##
==========================================
+ Coverage   46.91%   47.11%   +0.19%     
==========================================
  Files          17       17              
  Lines        1021     1021              
==========================================
+ Hits          479      481       +2     
+ Misses        542      540       -2     

Impacted Files	Coverage Δ
src/constructors.jl	83.01% <100.00%> (+3.77%)	⬆️

---

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 c929d0e...213c0aa. Read the comment docs.