Add cmdline interface for renderer options and add percent_of_total option

[joerick]

Adds a command-line option -p --render-option that allows arbitrary setting of render options.

For example, to show percentages on the command line, you can do pyinstrument -p time=percent_of_total myscript.py.

Processor options can also be set, using a JSON dict form, like pyinstrument -p 'processor_options={"filter_threshold": 0}' myscript.py, or dotted form, like pyinstrument -p processor_options.filter_threshold=0 myscript.py

Closes #180 and closes #174

[steinarvk-oda]

Seems like this would remove the need for #180 !