Add CLI args option to constructor and blameByFile method

[un-def]

let blamer = new Blamer('git', '-w -M');
blamer.blameByFile(file)            // git blame -w -M
blamer.blameByFile(file, '-M -C')   // git blame -M -C
blamer.blameByFile(file, '')        // git blame
blamer = new Blamer('git');
blamer.blameByFile(file)            // git blame -w

This PR closes Issue #2.

[coveralls]

Coverage increased (+0.3%) to 96.364% when pulling 397a920 on un-def:args-option into b90fbed on kucherenko:master.

[kucherenko]

thank you, PR merged and available at blamer@0.1.13