update existing plugins section of readme (adding superagent-mock)
update existing plugins section of readme (adding superagent-mock)

thejameskyle/superagent,noikiy/suberagent,puterakahfi/superagent,visionmedia/superagent,zsh2938/superagent,deltreey/superagent,eligolding/superagent,KingMario/superagent,stanzi83/superagent,noikiy/suberagent,soyuka/superagent,fredigato/superagent,zsh2938/superagent,soyuka/superagent,wushuyi/superagent,ertrzyiks/superagent,yeatszhang/superagent,focusaurus/superagent,kmalakoff/superagent-ls,mvanlonden/superagent-webpack-client,aaroncrows/superagent,focusaurus/superagent,craigspaeth/superagent,jarvisaoieong/superagent,simonvizzini/superagent,alinz/superagent,killerlayup/superagent,ertrzyiks/superagent,ccapndave/superagent,alinz/superagent,sutoiku/superagent,reinaldo13/superagent,drinks/superagent,targos/superagent,nemisj/superagent,nemisj/superagent,puterakahfi/superagent,brotchie/superagent,targos/superagent,sutoiku/superagent,thejameskyle/superagent,drinks/superagent,aaroncrows/superagent,stasha/superagent,RubenVerborgh/superagent,RubenVerborgh/superagent,andyburke/superagent,mvanlonden/superagent-webpack-client,geminiyellow/superagent,reinaldo13/superagent,wushuyi/superagent,h1bomb/superagent,craigspaeth/superagent,KuroGuo/suberagent,sebastianvera/superagent,brotchie/superagent,kmalakoff/superagent-ls,visionmedia/superagent,deltreey/superagent,stasha/superagent,geminiyellow/superagent,modulexcite/superagent,yeatszhang/superagent,visionmedia/superagent,brainbytes/superagent,yussan/superagent,simonvizzini/superagent,killerlayup/superagent,andyburke/superagent,yussan/superagent,eligolding/superagent,KuroGuo/suberagent,jarvisaoieong/superagent,brainbytes/superagent,ccapndave/superagent,KingMario/superagent,sebastianvera/superagent,modulexcite/superagent,stanzi83/superagent,h1bomb/superagent,fredigato/superagent