Respect err.showDiff, add Base reporter test (close #810)
only write to stdout in reporters
Less re-rendering with progress reporter

With a large number of small tests a significant portion of the testing time is simply re-rendering the progress bar, this only re-renders the progress bar if the progress has changed.
#888 Fix for --watch+only() issue
add support for reporter options
if a reporter has a .done method, call it before exiting
xunit reporter support for optionally writing to a file
Fix diffs of circular structures (close #1198, fix #1179)
Strip commas from the JSON.stringify output before diffing
Add tests for #1182 (close #1182)
Cleanup #1267
Update history.md
1.21.0
Suport ops path specify

Read specified file for configuration includes "--opts" CLI option.
By the default, read test/mocha.opts as before.
Update history.md
Fix getting timeout from Context (close #1282)
bumping to 1.21.1 for missing build
Revert "allow local modules to work as ui and reporters"

This reverts commit 5566faf9e7e823f75c140ba2cb84d7c97a2ce45b.
reverting "adding node_modules dir to use local modules for ui & reporters" (from cleanup).
bumping to 1.21.2