Introduce MatchSupportedRequestFn option for more granular decision making process to compress a response

[thekondor]

Rationale

WithMatchSupportedRequestFn option is introduced. This allows to make a decision about response compression basing on a combination of other headers and values from a request (like #42).

Changes

• No breaking changes in API for existing clients
• No breaking changes in behavior for existing clients
• WithMatchSupportedRequestFn() accepts a function with a signature: func(req *http.Request) (ok bool, supported bool). The first returned ok value is a marker that no other checks if a compression is excluded should be applied: supported is a decision for compression.
• MatchSupportedRequestFn has a precedence over Excluded{Extensions,Paths,PathesRegexs} lists. If ok is false, the other (= the mentioned before) exclusion rules are applied.

Notes

• http.Request is used intentionally instead of, say, gin.Context. The goal is to have a safe limited interface.
• In general, existing exclusion lists could be re-implemented in the future through the newly added interface.

[appleboy]

@thekondor Can you resolve conflicts?