Adds range settings for reading time estimate ranges. estimate_range to
control the range percentage ± and range_str for the string to combine the
low and high estimates.

Depending on the reading time estimate and the estimate range
percentage, we may end up with identical high and low estimates or
flooring out at zero minutes. Test for these cases and return a range
that makes sense.

…timate' into develop

…range_estimate' into develop"

This reverts commit 36d01e0, reversing
changes made to 8ce42a3.

This reverts commit 8ce42a3.

…-readingtime into develop