New feature request: do not merge comment lines

[YongLiedu]

Sometimes it would be better to ignore/don't merge comment lines.

avoid change something like this

# this is a sample.
# bash command and parameters here
# results

to

# this is a sample. bash command
# and parameters here results

[yihui]

Sounds like a reasonable feature request, but I don't have time to implement it. All I can say right now is that pull requests are welcome!

Also note roxygen comments #' will never be rewrapped.

[yihui]

Implemented via 4e31ffb. Thanks!