Add render exception config

[anhtrantuan]

Why is this necessary?

• Current implementation of swallowing error in Mjml::Parse#render method is dangerous. Particularly, it does not only produce confusing result, but also make it difficult to debug.

What were changed?

• Add exception raising configuration for Mjml::Parse#render method:

Mjml.setup do |config|
  # Default is `false` (errors suppressed), set to `true` to enable error raising
  config.raise_render_exception = true
end

[sighmon]

Awesome, thanks @anhtrantuan. I'll push the new version to ruby gems soon.