Adds RStudio addins

[JhossePaul]

This commit adds two addins to RStudio to bind shortcuts for easier usage. Now you can, for example, bind
Alt+Shift+L and lint the current source code.

[codecov-io]

Codecov Report

Merging #264 into master will decrease coverage by 0.7%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
- Coverage   86.12%   85.41%   -0.71%     
==========================================
  Files          39       40       +1     
  Lines        2183     2201      +18     
==========================================
  Hits         1880     1880              
- Misses        303      321      +18

Impacted Files	Coverage Δ
R/addins.R	0% <0%> (ø)

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[jimhester]

Probably would be safer to retrieve the project directory (rstudioapi::getActiveProject()) and pass that to lint_package()?

[JhossePaul]

Yep. Indeed I was thinking I completely forgot defensive programming. I'll add this and some more checks and then update the pull request

[jimhester]

Thanks this is a good idea! I made a minor comment, could you also add a note to NEWS.md with this change and mentioning your GitHub handle and this PR number?

Thanks!

[jimhester]

Great thanks!