Add option for specifying a minimum width of ordered lists

[edwar4rd]

See #464 .
I added ol_width in rendering options to specify the reserved width of ordered list markers.
For example, having ol_width = 5 give you 1. something and 123. something and 1234. something.
A single whitespace is always added, so ol_width <= 3 have no effect.
The previous behavior is equivalent to ol_width = 4.

Test tests::commonmark::commonmark_relist is adjusted with the changes.
During testing, self.prefix.len() - marker_width can sometimes cause a overflow.
I'm not too sure what do self.prefix mean, but

let new_len = if self.prefix.len() > marker_width {
    self.prefix.len() - marker_width
} else {
    0
};

seemed to work fine.