add chunk addin

[EmilHvitfeldt]

This PR adds an RStudio addin that adds a decorate() chunk after a named chunk when selected. Reference: https://twitter.com/Emil_Hvitfeldt/status/1314836919797202944

The function will:

• Error if a whole chunk isn't fully selected
• Error if chunk is unnamed
• Add include = FALSE to selected chunk (this has been updated since the first gif)
• Place a new chunk after selected chunk with chunk name original_flair

If more then 1 chunk is selected only the first chunk will be affected.

[kbodwin]

This is super cool, and I'm so sorry I missed it for so long! Thank you so much!