Allow selection of an encoding on slurp/spew in Mojo::File

[genio]

Summary

Alter Mojo::File to:

• Allow slurp and decode with $file->slurp('UTF-8'); etc.
• Allow spew and encode with $file->spew('contents', 'UTF-8'); etc.

Motivation

As we discussed on IRC, allowing these options not only makes it easier for users as well as bring things more inline with what is allowed in the Mojo.js framework.

[genio]

@Grinnz Correct. Since spurt can take an array of content, we can't easily check the last argument. Discussion was that spurt would likely be done away with in time

[karenetheridge]

This is great; thank you!