Support disabling specific validation

[mtsmfm]

Thank you for an awesome gem 🌟

I propose configuration to disable specific validation.
I want to disable only maxlength validation.

I'm not too sure about the API to disable.
Please give me feedback if you have ideas.

[mtsmfm]

Woops, build failed 😵

But it seems that we can't see details about the build.
Could you enable Travis for this repository?

[mtsmfm]

I noticed that we should support ruby 1.8 and I used 1.9 Hash syntax 😓
Fixed and force pushed 😄

[amatsuda]

Whoa! I wasn't aware of the Travis status... Thanks for letting me know that!

I haven't looked deeper into the code yet, but before going any further, I'd like to know your use case.
Why would you want to partially disable some validations in this way?

[mtsmfm]

I want to disable maxlength only.
There are two reasons:

1. It seems that HTML5 maxlength counts emoji (like 🌋) as 2 characters.

   http://www.wufoo.com/html5/attributes/03-maxlength.html

2. User may not notice that some chars are dropped when using copy & paste.

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I implemented options for all validations for symmetry.
I don't have any situations for another validations.