Add :html_formats option

[timfjord]

Closes #13.

.eex templates are compiled with Phoenix.HTML.Engine only when the encoder of their format is Phoenix.HTML.Engine. A format with its own encoder, such as MJML emails compiled to HTML, falls back to EEx.SmartEngine and interpolates values unescaped.

This adds the :html_formats option, as discussed in the issue:

config :phoenix_template, :format_encoders, mjml: MyApp.MJML
config :phoenix_template, :html_formats, [:mjml]

Templates of the listed formats are compiled exactly like .html.eex ones: values are escaped unless marked as safe, :trim_on_html_eex_engine applies, and the format encoder receives the {:safe, iodata} tuple.

Nothing changes for apps that do not set the option. The error raised when phoenix_html is missing now names the actual format instead of .html.eex.

[josevalim]

💚 💙 💜 💛 ❤️