LinkGenerator: allow semicolon destination prefix

[dakujem]

• fixes bug #255
• BC break? no

Allows passing semicolon-prefixed destinations to LinkGenerator::link calls:

(new LinkGenerator(...))->link(':Foo:Bar:baz');
(new LinkGenerator(...))->link(':Bar:baz');

// and also this:
(new LinkGenerator(...))->link($presenter->getAction(true));

-- closes #255

[dakujem]

@dg I'd be grateful if this was merged & released promptly. It would be awesome. I see praparations for next release. This should be fully compatible.

[dakujem]

Should I rebase this? Any plan to fix the issue?

[dakujem]

You should update your test config - PHP 7.1 is no longer your target.

[dakujem]

Do we really need to force-push to master? Isn't that what dev branches are meant for? I've needed to rebased this PR 4 times since I originally prepared it. It would be awesome to accept it already.