Refactoring API and organizing code

[LandonTClipp]

Two things are being done here:

1. The code is being refactored to be a very thin layer around the
   underlying filesystems, and to match their API as close as possible.
2. Organizing the code to make my design philosophy more clear and
   concise.

[LandonTClipp]

Make RelativeTo return non-nil object on errors.

Also, ensure the error message makes sense when RelativeTo breaks because of bad arguments.

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 0.67%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   60.00%   60.67%   +0.67%     
==========================================
  Files           1        1              
  Lines         170      178       +8     
==========================================
+ Hits          102      108       +6     
- Misses         56       63       +7     
+ Partials       12        7       -5     

Impacted Files	Coverage Δ
path.go	60.67% <71.42%> (+0.67%)	⬆️

---

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 f7fc246...4ad2575. Read the comment docs.