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
File handling was broken on Windows. This commit contains a revision of the path handling with separation of file paths and urls where needed.
There may be remaining issues and there may be better ways to do this, but it is easier to start that refactoring job with a set of passing tests.
Fixes#687Fixes#660
