Change progress_mark with increment

[noctivityinc]

Current Problem

This is a sweet gem. The only issue is that the progress_mark is always the same, which isn't always helpful.

For example I will get this

======= 7 ===== ETA: XX:XX:XX

But there are often times that I want to see if there was an issue, and for that I'd like to see a different mark, such as:

==x==== 7 ======

Potential Solution or New Behavior

Simple. I'd like to be able to do

bar.increment('x')

which would output that mark if specified on the progress bar.

Mockups

==x==== 7 ======