Add image extension method for save as tga

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR adds SaveAsTga extension method.

[codecov]

Codecov Report

Merging #1155 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1155   +/-   ##
=======================================
  Coverage   82.23%   82.23%           
=======================================
  Files         680      681    +1     
  Lines       29301    29305    +4     
  Branches     3279     3280    +1     
=======================================
+ Hits        24096    24100    +4     
  Misses       4516     4516           
  Partials      689      689           

Flag	Coverage Δ
#unittests	82.23% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Tga/ImageExtensions.cs	100.00% <100.00%> (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 6682529...e80a6e3. Read the comment docs.