Added --trace-plugins command-line option.  Fixed recent crasher in startup logic when leoSettings.leo does not exist.

Leo build: 20140918062023
Fixed crasher in leoBridge.py when run, as is usual, with a null gui.

Leo build: 20140918062236
Sorry, the previous commit message is wrong.  It is the bookmark.py plugin that used to crash when run with a null gui, say from the Leo bridge.

Leo build: 20140918062622