Add 3-way merge implementation

[satabin]

No description provided.

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by 1.65%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   39.43%   37.77%   -1.66%     
==========================================
  Files          43       44       +1     
  Lines        1481     1546      +65     
  Branches      156      158       +2     
==========================================
  Hits          584      584              
- Misses        897      962      +65

Impacted Files	Coverage Δ
src/main/scala/gnieh/diff/Diff3.scala	0% <0%> (ø)

---

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 250f3c2...978ea54. Read the comment docs.