Recognize common attributes in the PR title

[wilzbach]

While a contributor can't set labels himself, he could do so via the PR title. Some are already commonly used:

• [Trivial] -> add "Trivial" label

To avoid any security issues the bot would:

• only add labels not remove them
• only used a fixed set of label

Proposed list (for the beginning):

• Trivial
• Doc
• WIP

Multiple labels could be provided with a comma, e.g. "[Trivial, Doc] A fancy title" though I doubt that this will be a common case.