Website/URL check as filter

[ghost]

Potential use case: Check if a URL is still online or contains keywords, matches a regex etc.
Examples:

• check if http://example.com/server-status is reachable (status code == 200)
• check for existence of HTTP headers

Possible functions and parameters:

• Check the status code. Match it against integer, but matching against a regex is also useful (e.g. 4..)
• Check the header against a regex
• check the body against a regex
• possibility to negate the result

If all checks succeed, sent the event, otherwise drop it.

[ghost]

fixed by #1789 and #1811