Add rcm step

[9999years]

https://github.com/thoughtbot/rcm

Tested on my machine with cargo run -- --only rcm.

Should I add an #[serde(alias = "rcup")] on the Step item? (The tool suite is called rcm, and rcup is the one that actually updates the dotfiles.)

[codecov]

Codecov Report

Merging #91 (adde528) into master (9cd155f) will decrease coverage by 0.14%.
The diff coverage is 0.00%.

❗ Current head adde528 differs from pull request most recent head 9e1fba6. Consider uploading reports for the commit 9e1fba6 to get more accurate results

@@            Coverage Diff            @@
##           master     #91      +/-   ##
=========================================
- Coverage    3.50%   3.35%   -0.15%     
=========================================
  Files          27      27              
  Lines        3141    3279     +138     
=========================================
  Hits          110     110              
- Misses       3031    3169     +138     

Impacted Files	Coverage Δ
src/config.rs	0.00% <0.00%> (ø)
src/main.rs	0.41% <0.00%> (-0.01%)	⬇️
src/steps/containers.rs	0.00% <0.00%> (ø)
src/steps/emacs.rs	0.00% <0.00%> (ø)
src/steps/generic.rs	0.00% <0.00%> (ø)
src/steps/git.rs	0.00% <0.00%> (ø)
src/steps/kakoune.rs	0.00% <0.00%> (ø)
src/steps/node.rs	0.00% <0.00%> (ø)
src/steps/os/archlinux.rs	0.00% <0.00%> (ø)
src/steps/os/linux.rs	22.29% <0.00%> (-1.01%)	⬇️
... and 9 more

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