Build errors and Typescript changes

[VagnerNico]

Hi!

Thanks for the implementation.
I'm trying to use this Component on my App, and I get some errors about missing types declaration.
I made a fork and changed all files to Typescript, and I'd noticed that the build seems to be broken, including on the master branch.
Also, I want to add a feature to change the overlay opacity gradually with the dragged height.
Could you fix the Gulp build and add support for Typescript?
So, I can make a PR with these mentioned changes.