acai-cli: add --patterns flag

[MartinHelmut]

Like already mentioned in #2 there should be a flag to configure the pattern(s) used to match the commit messages (--patterns / -p). This would enable better results if commit messages are strictly defined. Usage could be e.g.:

$ acai --patterns "^fix\(.*\):"

There has to be defined if this should be some kind of pattern matcher or regular expression. Currently the following regex is default:

/^(?:(?!branch.+into 'master').)*\bfix(?:ed|es)?|close(?:s|d)?\b/i

The default regex excludes merge commits and looks for commits including: "fix", "fixed", "fixes", "close", "closed", "closes".

The possibility to pass a regex to the acai.scanner() is already given.