CppLint: Make "root" flag configurable via CPPLINT.cfg

[gueraf]

Enables CppLint to pick up the "root" flag from a CPPLINT.cfg file.

Practical use: The CppStyle plugin for Eclipse, for example, doesn't allow passing flags to CppLint. Using CPPLINT.cfg files comes in handy here.

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[gueraf]

I signed it!

[googlebot]

CLAs look good, thanks!

[tkruse]

I added this PR to my cpplint fork as tkruse/cpplint#30

[wimrijnders]

Thanks, guys! I'll take it for a spin again as soon as I can get the change.

[eglaysher]

I do apologize for the delay in getting to this; previously, no one was watching this repository. This looks straightforward enough. Thank you for your contribution.