Show warning when loadTask takes too long.

grunt --help and cli works only well if the lookup for all available tasks is done within a comfortable 100ms. (different for every developer. I am just assuming some time here) Unfortunately even registering some tasks takes a lot longer than a second. It would be good if at least the developer would be informed that the task he is writing takes too long to load.

Since there are plenty plugins out there, it might be even better if a message would be shown to users in case a task takes longer than 10ms to load (allowing for 10 tasks to be registered in 100ms). This way the users can in turn annoy developers with bug reports if a task takes too long to load.

Of course it should better be possible to switch off the message for users on a slow machine. ;)

Start by tracing the task lookup and registration path exercised by `grunt --help`; the issue does not name a file or test. Determine where load duration can be measured, then define warning thresholds and an opt-out for slow machines. Done means slow task loading is reported without disrupting normal CLI use.

Written by the indexing model from the issue text.