Add root option to CPPLINT.cfg

[wimrijnders]

I have a bunch of related, non-git based projects. Most but not all share the same root directory. It's tiresome to keep on adding the --root option on the command line.

I would appreciate it if this field can be specified in the CPPLINT.cfg file.

[gueraf]

#129

[wimrijnders]

Closing because moved to other issue.