Add different formats

[aswinkarthik]

Fixes #14

Formats added:

1. git diff style (default now)
2. git diff --color-words
3. git diff --word-diff
4. Updated JSON with original row for modifications
5. Old formats are retained

[codecov[bot]]

Codecov Report

Merging #22 into master will increase coverage by 11.01%. The diff coverage is 83.65%.

@@            Coverage Diff             @@
##           master     #22       +/-   ##
==========================================
+ Coverage   62.59%   73.6%   +11.01%     
==========================================
  Files           8      11        +3     
  Lines         262     413      +151     
==========================================
+ Hits          164     304      +140     
- Misses         94      97        +3     
- Partials        4      12        +8

Impacted Files	Coverage Δ
cmd/version.go	0% <ø> (ø)	:arrow_up:
pkg/digest/file_digest.go	100% <100%> (ø)
pkg/digest/config.go	100% <100%> (ø)
cmd/config.go	89.47% <100%> (+2.8%)	:arrow_up:
cmd/root.go	10.29% <14.28%> (-0.65%)	:arrow_down:
pkg/digest/digest.go	87.03% <61.11%> (+3.7%)	:arrow_up:
pkg/digest/engine.go	71.42% <71.42%> (ø)
pkg/digest/diff.go	89.18% <89.18%> (ø)
cmd/formatter.go	91.22% <92.03%> (+6.61%)	:arrow_up:
... and 4 more