Useful if using or sharing files with users that use editors that
append a unicode byte order marker header (like Windows notepad).
This will still assume files are UTF-8 encoded.
Closes#2075
If Page.Markup was not set by the user, it will now be set after
guessing from the file extension. This means, Page.Markup will be set in
any case. It can be used by a theme to differentiate between markup
types.
Fixes#1950
Meny people, including me, have a custom robots.txt in static.
Also remove that option from the command line; it doesn't feel
important enough.
Fixes ##2049
This also includes a refactor of the hugofs package and its usage.
The motivation for that is:
The Afero filesystems are brilliant. Hugo's way of adding a dozen of global variables for the different filesystems was a mistake. In readFile (and also in some other places in Hugo today) we need a way to restrict the access inside the working dir. We could use ioutil.ReadFile and implement the path checking, checking the base path and the dots ("..") etc. But it is obviously better to use an Afero BasePathFs combined witha ReadOnlyFs. We could create a use-once-filesystem and handle the initialization ourselves, but since this is also useful to others and the initialization depends on some other global state (which would mean to create a new file system on every invocation), we might as well do it properly and encapsulate the predefined set of filesystems. This change also leads the way, if needed, to encapsulate the file systems in a struct, making it possible to have several file system sets in action at once (parallel multilanguage site building? With Moore's law and all...)
Fixes#1551
It is a fairly costly operation:
```
benchmark old ns/op new ns/op delta
BenchmarkScratchGet-4 109 31.6 -71.01%
benchmark old allocs new allocs delta
BenchmarkScratchGet-4 0 0 +0.00%
benchmark old bytes new bytes delta
BenchmarkScratchGet-4 0 0 +0.00%
´´´
