Add transform anchor text

[pablolmiranda]

Implementation to solve the problem describe on #40

[tasti]

Support for transforming anchor text will be available in v1.0.0. For example:

const textDecorator = (text) => 'See this repo on Github';

<Linkify textDecorator={textDecorator}>
  ...
</Linkify>

Any feedback is appreciated.

[dpup]

Thanks!