Implements the includedContentTypes option for the compress middleware

docs/content/middlewares/http/compress.md

@@ -58,7 +58,7 @@ http:
     If the `Accept-Encoding` request header is absent, the response won't be encoded.
     If it is present, but its value is the empty string, then compression is disabled.
     * The response is not already compressed, i.e. the `Content-Encoding` response header is not already set.
-    * The response`Content-Type` header is not one among the [excludedContentTypes options](#excludedcontenttypes).
+    * The response`Content-Type` header is not one among the [excludedContentTypes options](#excludedcontenttypes), or is one among the [includedContentTypes options](#includedcontenttypes).
     * The response body is larger than the [configured minimum amount of bytes](#minresponsebodybytes) (default is `1024`).
 
 ## Configuration Options
@@ -73,6 +73,10 @@ The responses with content types defined in `excludedContentTypes` are not compr
 
 Content types are compared in a case-insensitive, whitespace-ignored manner.
 
+!!! info 
+
+    The `excludedContentTypes` and `includedContentTypes` options are mutually exclusive.
+
 !!! info "In the case of gzip"
 
     If the `Content-Type` header is not defined, or empty, the compress middleware will automatically [detect](https://mimesniff.spec.whatwg.org/) a content type.
@@ -117,6 +121,59 @@ http:
     excludedContentTypes = ["text/event-stream"]
 ```
 
+### `includedContentTypes`
+
+_Optional, Default=""_
+
+`includedContentTypes` specifies a list of content types to compare the `Content-Type` header of the responses before compressing.
+
+The responses with content types defined in `includedContentTypes` are compressed. 
+
+Content types are compared in a case-insensitive, whitespace-ignored manner.
+
+!!! info
+
+    The `excludedContentTypes` and `includedContentTypes` options are mutually exclusive.
+
+```yaml tab="Docker & Swarm"
+labels:
+  - "traefik.http.middlewares.test-compress.compress.includedcontenttypes=application/json,text/html,text/plain"
+```
+
+```yaml tab="Kubernetes"
+apiVersion: traefik.io/v1alpha1
+kind: Middleware
+metadata:
+  name: test-compress
+spec:
+  compress:
+    includedContentTypes:
+      - application/json
+      - text/html
+      - text/plain
+```
+
+```yaml tab="Consul Catalog"
+- "traefik.http.middlewares.test-compress.compress.includedcontenttypes=application/json,text/html,text/plain"
+```
+
+```yaml tab="File (YAML)"
+http:
+  middlewares:
+    test-compress:
+      compress:
+        includedContentTypes:
+          - application/json
+          - text/html
+          - text/plain
+```
+
+```toml tab="File (TOML)"
+[http.middlewares]
+  [http.middlewares.test-compress.compress]
+    includedContentTypes = ["application/json","text/html","text/plain"]
+```
+
 ### `minResponseBodyBytes`
 
 _Optional, Default=1024_