In Target Markdown interactive mode, print the return value if tar_simple is TRUE

[wlandau]

Prework

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Description

Should make it easier to prototype simple targets.

[wlandau]

Also print out "converted to target" later on in the output.

[wlandau]

Not actually sure if this is possible in knitr for a custom language engine.

[wlandau]

@cderv pointed out that users can simply switch back to the {r} engine when prototyping. So I think the {targets} engine can stick with the current behavior.

[jmbuhr]

oh, this is clever!