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).
