Support abbr options and refine the code

DESC:
[Feature]
1. Support abbr options and refine the code
(1). Support abbr options, for example: :seed -> :s
(2). Refine the code, use block to reduce the duplicated codes.
(3). Update the README, use the new hash style and focus on the format
feature.
(4). Discard the "get" and "strf" interface, use "gen" instead.
(5). Add check option feature.
Merge branch 'develop' of github.com:sibevin/random_token into develop

Conflicts:
	lib/random_token.rb