Support for string "template_lookup"

[sharpaper]

When defining my own template function

functools.partial(template, template_adapter=..., template_lookup=..., template_settings=...)

template_lookup must be a list. This was not really clear, until I looked at the Bottle code. It would be useful to support string values as well for when there is only one template folder, like this template_lookup="./templates".

Or maybe add this information to the documentation.