Add support for overriding accepted format

[stempler]

Support overriding accepted format for a request by specifying a query parameter or adding an extension to the request URL:

• if a query parameter "accept" is provided, its value is looked up in the supported list of extension, if not present it is directly used as media type
• otherwise, if the request URL ends on . plus a supported extension, the media type associated to the extension is used as media type
• any values provided via the Accept header are appended

See #76. Follows priority for format/encoding as suggested by @stephanr

[stempler]

Related to the query parameter it was rather arbitrary that I chose accept from the proposals in #76. Just saw that for some requests f seems to be already supported. Would be no problem to change it to use f generally or to support both. Any preferences?

[stempler]

Added the f query parameter now also to the generic format override support, so it can be consistently used with the resources that already handle it and those that don't. The logic for the cases where the parameter is already specifically handled takes precedence over the generic mechanism introduced in the PR.