Clean formulas command

[henriquemoraes8]

- What I did

• added rit clean formulas command that cleans ~/.rit/formulas folder

• Created a utilitary method in stdin to overwrite the os.Stdin and allow for stdin tests

• Proposed a stdin test for increased command coverage

- How to verify it
run rit clean formulas and verify your saved formulas

- Description for the changelog
added rit clean formulas command to cleanup downloaded formulas

[codecov-commenter]

Codecov Report

Merging #288 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #288   +/-   ##
=======================================
  Coverage   69.34%   69.34%           
=======================================
  Files          73       73           
  Lines        2799     2799           
=======================================
  Hits         1941     1941           
  Misses        624      624           
  Partials      234      234           

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