Add support for executing truffle locally

[mattaereal]

Adds support to be able to run truffle compile without having to install it globally.

Closes #105

Truffle being used by default:

matt@delleite:contracts/ (master*) $ slither .                                                                                                                [18:11:18]
INFO:Slither:truffle compile running...
ERROR:root:Error in .
ERROR:root:Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/slither_analyzer-0.3.1-py3.7.egg/slither/__main__.py", line 229, in main_impl
    (results, number_contracts) = process_truffle(filename, args, detector_classes, printer_classes)
  File "/usr/lib/python3.7/site-packages/slither_analyzer-0.3.1-py3.7.egg/slither/__main__.py", line 64, in process_truffle
    process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
  File "/usr/lib/python3.7/subprocess.py", line 769, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.7/subprocess.py", line 1516, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'truffle': 'truffle'

With truffle locally (--truffle-local):

matt@delleite:contracts/ (master*) $ slither . --truffle-local                                                                                                [18:11:57]
INFO:Slither:truffle compile running...                                                                                                                                         
INFO:Slither:                                                                                                                                                                   
INFO:Detectors: 
....
INFO:Slither:. analyzed (10 contracts), 48 result(s) found

[CLAassistant]

All committers have signed the CLA.

[montyly]

Thanks @mattaereal for your contribution!

Could you sign the CLA?

Is there any potential issue to always run npx truffle? We can simply set it as the default command

[mattaereal]

Signed, and not that I'm aware of. But it will prioritize the local installation over the global one, if that's what you want.

A good brief comparison.