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
Using it for list pages doesn't work and has potential weird side-effects.
The user probably meant to range over .Site.ReqularPages, and that is now marked clearly in the log.
This is a pretty fundamental change in Hugo, but absolutely needed if we should have any hope of getting "multiple outputs" done.
This commit's goal is to say:
* Every file target path is created by `createTargetPath`, i.e. one function for all.
* That function takes every page and site parameter into account, to avoid fragile string parsing to uglify etc. later on.
* The path creation logic has full test coverage.
* All permalinks, paginator URLs etc. are then built on top of that same logic.
Fixes#1252Fixes#2110Closes#2374Fixes#1885Fixes#3102Fixes#3179Fixes#1641Fixes#1989
