Suggestion: Allow tweaking or overriding the view (theme)

It would be nice if there was an option to either completely customize the better_errors page, or maybe at least just to tweak it (whichever is easier to implement).

**Tweaking approach for example:**
Enable some settings that can be placed in the initializer to set some colors, and font size.
Specifically, I would like to change the colors of the black header and increase font size a little.

**Overriding view:**
This can be done in several ways I guess, one that I saw elsewhere that was working nicely, is to add a generator (like `rails generate better_errors:views`) that will generate view files in the views folder, and then anyone can tweak as they want.

EDIT:
I just noticed that there is a commented out light theme section in the main template, so someone has already done half the work. I tried doing the other half of allowing it to be configurable, but realized I am in over my head... didn't know what is the proper way to pass variables from the middleware to the template (tried with `env` but realized I am doing something wrong...).

For now, I just commented out this block in [my form](https://github.com/charliesome/better_errors/compare/master...DannyBen:light-theme)