configuration for a subject prefix for email

[francescob]

Added an option to add a subject prefix to the emails sent by solid_errors. Useful when you have multiple apps/environments using solid errors and what to easily filter the email received

[fractaledmind]

I like the idea, just one small change needed

[fractaledmind]

Handle an empty prefix

[francescob]

Ok, now the empty prefix is handled.
i also have another idea that fits in the way we use solid_errors:
Make subject_prefix enabled by default, with a value of
[#{Rails.application.name.upcase][#Rails.application.env.upcase].
Disable it with config.solid_errors.subject_prefix = "" or override it with your own custom string
What do you think about that?