provided api-level control over line color, and added dashed line for median pre to median post
added control over line color to command line interface, also improved first usage example as the previous ymin/ymax range wasn't good for these data
updated usage example results
made plot background transparent. this addresses #1174
Merge pull request #1196 from gregcaporaso/issue-1174

Issue 1174: enhancements to identify_paired_difference plots