Use FormsyText internal _value to allow external setValue calls

[astonm]

Based on the example at christianalfoni/formsy-react#180

Fixes #161

[codeaholicguy]

@rojobuffalo @astonm as this fix, we will face with problem that text field wont be updated immediately if prop updateImmediately is on in 0.5.4.

Because

    if (this.props.updateImmediately) {
       if (!this.changeValue) {
         this.changeValue = debounce(this.setValue, 400);
       }
       this.changeValue(event.currentTarget.value);
     }

I think we should control value instead of using getValue.

[mbrookes]

TBH based on a cursory review, I'm not sure this PR should have been merged (buggy or not) as the parent component should be setting props, rather than using imperative methods.

[codeaholicguy]

@mbrookes related issue #191