+ Adds parsing of contextual ui sections to aerc config.
+ Add GetUiConfig method for AercConfig that is used to get the
  specialized UI config.
+ Add UiConfig method to AccountView to get specialized UI Config.
+ Modifies Aerc codebase to use specialized UIConfig instead.
+ Adds documentation for Contextual UI Configuration

Before, while the docs stated that template-dirs was a colon-separated
list, a delimiter was not specified in the struct tag, so it was falling
back to the default for the ini library (a comma). Also added a note to
the docs to clarify that templates are configured in the [templates]
section.

The functionality was broken since the decoding changes.
This commit also simplifies the code (in my view) to make the application logic
easier to follow.
The docs are updated accordingly (the feature was poorly documented).

As far as I am aware there should be no breaking changes (and is certainly
still in the spec of the prior documentation)