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
Add validation before creating aliases:
* Prevent creating aliases outside webroot (public/ dir)
* Skip empty "" alias
* Skip "/" → "/index.html", which gets overwritten anyway
* Refuse to create Windows-invalid filenames on Windows;
warn on other platforms
* In case of invalid aliases, after skipping them,
return `err = nil` to prevent the error passing up
all the way to `hugolib.Render()` and causing Hugo to abort.
* Update alias tests.
Fixes#701: Add support for alias with whitespace
Fixes#1418: Add validation for alias
I want to move all logic to writing aliases to target so I can pave the
way for writing aliases specific to other runtimes (like .htaccess for
apache or a script for updating AWS or symlinking on a filesystem).
