feat: control which linefeed character to use

[merceyz]

What:
Allow the user to control which linefeed character to use.

Why:
Babel does not care about line endings and always uses \n, which means you're forced to keep the output files in the same format for fixtures to work. Which in a repository where core.autocrlf = true means extra config files and mixing the linefeed used in the repository. This is an unnecessary process that can be eliminated.

How:
After babel has transformed the code run a regex to replace the line endings with either \n, \r\n, the system default, or preserve from input, depending on what the user specifies.

[codecov-io]

Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #45   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         132    143   +11     
  Branches       33     35    +2     
=====================================
+ Hits          132    143   +11

Impacted Files	Coverage Δ
src/index.js	100% <100%> (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 1d22086...3b5e5ce. Read the comment docs.

[kentcdodds]

Looks good. Thanks!