Before this commit, Hugo used `html/template` for all Go templates.
While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.
This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.
A couple of notes:
* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.
Fixes#3221
This relates to #3123.
The interfaces and types in `target` made sense at some point, but now this package is too restricted to a hardcoded set of media types.
The overall current logic:
* Create a file path based on some `Translator` with some hardcoded logic handling uglyURLs, hardcoded html suffix etc.
* In in some cases (alias), a template is applied to create the alias file.
* Then the content is written to destination.
One could argue that it is the last bullet that is the actual core responsibility.
This commit fixes that by moving the `hugolib`-related logic where it belong, and simplify the code, i.e. remove the abstractions.
This code will most certainly evolve once we start on #3123, but now it is at least possible to understand where to start.
Fixes#3123
