Add support for symlinking plugins if available
Add support for symlinking plugins if available

Adds support for the (new in 3.9) automatic detection and handling of symlinked plugins.

Related to wp-cli/wp-cli#1015
aaemnnosttv/wp-cli,2ndkauboy/wp-cli,xwp/wp-cli,GaryJones/wp-cli,anttiviljami/wp-cli,a7127cff69cab32e5c4723e8446e21c4b7325dc/wp-cli,gitlost/wp-cli,rodrigoprimo/wp-cli,borekb/wp-cli,dpq/wp-cli,spiritedmedia/wp-cli,trepmal/wp-cli,GaryJones/wp-cli,trepmal/wp-cli,miya0001/wp-cli,macbookandrew/wp-cli,spiritedmedia/wp-cli,here/wp-cli,gedex/wp-cli,borekb/wp-cli,anttiviljami/wp-cli,gitlost/wp-cli,pressednet/wp-cli,staylor/wp-cli,heiglandreas/wp-cli,Yas3r/wp-cli,here/wp-cli,aaemnnosttv/wp-cli,rodrigoprimo/wp-cli,anttiviljami/wp-cli,shawnhooper/wp-cli,staylor/wp-cli,miya0001/wp-cli,aaemnnosttv/wp-cli,a7127cff69cab32e5c4723e8446e21c4b7325dc/wp-cli,macbookandrew/wp-cli,gitlost/wp-cli,xwp/wp-cli,JohnDittmar/wp-cli,Yas3r/wp-cli,JohnDittmar/wp-cli,nyordanov/wp-cli,dpq/wp-cli,staylor/wp-cli,heiglandreas/wp-cli,2ndkauboy/wp-cli,shawnhooper/wp-cli,duncanjbrown/wp-cli,GaryJones/wp-cli,rohmann/wp-cli,boonebgorges/wp-cli,blueblazeassociates/wp-cli,gedex/wp-cli,nyordanov/wp-cli,duncanjbrown/wp-cli,ahmadawais/wp-cli,gedex/wp-cli,trepmal/wp-cli,nyordanov/wp-cli,boonebgorges/wp-cli,blueblazeassociates/wp-cli,shawnhooper/wp-cli,rohmann/wp-cli,2ndkauboy/wp-cli,a7127cff69cab32e5c4723e8446e21c4b7325dc/wp-cli,dpq/wp-cli,xwp/wp-cli,boonebgorges/wp-cli,macbookandrew/wp-cli,blueblazeassociates/wp-cli,miya0001/wp-cli,rodrigoprimo/wp-cli,duncanjbrown/wp-cli,pressednet/wp-cli,pressednet/wp-cli,JohnDittmar/wp-cli,spiritedmedia/wp-cli,Yas3r/wp-cli,ahmadawais/wp-cli,rohmann/wp-cli,borekb/wp-cli,ahmadawais/wp-cli