Ensure explicit targets always override browsers key targets

[existentialism]

Fixes #345.

[codecov-io]

Codecov Report

Merging #346 into master will decrease coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
- Coverage    93.7%   93.65%   -0.05%     
==========================================
  Files           6        6              
  Lines         254      252       -2     
  Branches       69       68       -1     
==========================================
- Hits          238      236       -2     
  Misses         11       11              
  Partials        5        5

Impacted Files	Coverage Δ
src/targets-parser.js	96.22% <100%> (-0.14%)	⬇️

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