support registering paths programmatically

[vishna]

closes #6

[codecov-io]

Codecov Report

Merging #24 into master will increase coverage by 0.85%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage    82.1%   82.95%   +0.85%     
==========================================
  Files          13       13              
  Lines         380      399      +19     
==========================================
+ Hits          312      331      +19     
  Misses         68       68

Impacted Files	Coverage Δ
lib/src/router.dart	84.94% <100%> (+3.36%)	⬆️
lib/src/voyager.dart	100% <100%> (ø)	⬆️

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