Do not show progress bar if not interactive or knitr in progress

Checking a NULL value for true results in an error

The previous commit correctly did not print the progress bar when
knitting, but it also threw an error when not knitting, this commit
fixes the issue in both cases

Add note to news

Correct issue number

Move progress bar interactive use to latest news block

Checking a NULL value for true results in an error

The previous commit correctly did not print the progress bar when
knitting, but it also threw an error when not knitting, this commit
fixes the issue in both cases

Add note to news

Correct issue number
Remove extraneous news item
Merge pull request #429 from jimhester/non_interactive_progress_bar

Do not show progress bar if not interactive or knitr in progress