Add static helper method

[F43nd1r]

The code in the readme is really just boilerplate code, which could be easily extracted in a static helper, as I just did here. I also adjusted the sample to show how it's used.

[spacecowboy]

I like it!

Couple of things:

• will likely conflict with
  Fixed FileUriExposed error on Nougat properly #118 which I'm
  planning on merging any day now.
• the method should be in AbstractFilePickerActivity because it's general
  functionality
• some tests would be nice (but I can add those if that's problematic)

[F43nd1r]

I haven't looked at the beta yet. I'll definetly wait for the merge and then check back how this can fit with it.
I wasn't sure about the class, as a static method like this could be anywhere, so I just used the only class I already had in my imports.

One question: Are there any resources how to use this library to access files which can only be reached through the SAF?

[spacecowboy]

The conflict is my problem though so don't worry about that point

[spacecowboy]

@F43nd1r you mean like physical SD cards? That is still an open question. There doesn't seem to be a way to access them outside of SAF.

[spacecowboy]

Sorry for the delay @F43nd1r . I just kind of forgot about this PR.

Rebased and moved this functionality in #138