Add support for customizable defaults (eg. make view alpha configurable)

[cjwirth]

The way the code is currently written, if you show a toast, it will always be completely opaque -- the alpha is always 1.

Originally, I thought passing in the alpha value might be a good way to do it, but there are some other values that might be nice to configure (basically, many of the static const values). It might be nice to add support to customize these values.

I thought of a few ways this could be done. The easiest might be to read from UserDefaults to see if there is a value to use instead of the hardcoded value. If not, it uses the original value.

[1in9ui5t]

Specifying the text alignment of toast title would be another nice to have.

[scalessec]

I've thought about doing this, but I don't like the complexity it adds. There are more robust Toast-like libraries out there if people need more runtime customization. That being said, it should be easy enough to fork and add if someone really wants these features at runtime.

[scalessec]

Custom styling is officially supported in Toast 3.0.