add man trans and clean title option to cars

[genkimind]

Added additional filters to use when dealing with cars. manual_transmission and clean_title to be set to 1 as appropriate.

[rbk]

I hope this gets merged. Great addition! For now I'll just copy the commit code to my project.

[juliomalegria]

This is already covered with the list_filters added a while back. I've updated the documentation with a example for cars/trucks.

An example of usage would be:

from craigslist import CraigslistForSale
CraigslistForSale(site='sfbay', category='cta', filters={
    'auto_transmission': ['manual'],
    'auto_title_status': ['clean']
})
# ...