fix(cli): make help match default element list

[kaicarver]

the cli usage doc did not reflect the current default list of elements localized:

it was:

$ s18n help extract
   ...
    -e, --elements [elems]    html elements to localize [default: 'title,p,h1,h2,h3,h4,h5,h6' | none: '.']

but the default in lib/extract.js is:

options.elements = arrayify(options.elements) || ['title', 'h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'p', 'small', 'a', 'button'];

[coveralls]

Coverage remained the same at 100.0% when pulling f985dbe on kaicarver:master into a1fecfc on bitjson:master.

[bitjson]

Looks great, thanks!