feat: Support multiple configuration files (#389)

* Allow configuration to be distributed

* catch iteration errors when collecting config files

* rm unused func

* Fix suffix check for config loading

* test configuration loading

* GATUS_CONFIG_PATH can be a file or a directory now

* Add deprecation note

* Fix cs

Co-authored-by: TwiN <twin@linux.com>

* cs fixes

Co-authored-by: TwiN <twin@linux.com>

* cs fixes

Co-authored-by: TwiN <twin@linux.com>

* cs fixes

Co-authored-by: TwiN <twin@linux.com>

* cs + rm useless line

Co-authored-by: TwiN <twin@linux.com>

* Update config/config.go

Co-authored-by: TwiN <twin@linux.com>

test-conf/conf.d/endpoints.yaml

@@ -0,0 +1,24 @@
+endpoints:
+  - name: front-end
+    group: core
+    url: "https://twin.sh/health"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"
+      - "[BODY].status == UP"
+      - "[RESPONSE_TIME] < 150"
+
+  - name: back-end
+    group: core
+    url: "https://example.org/"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"
+      - "[CERTIFICATE_EXPIRATION] > 48h"
+
+  - name: monitoring
+    group: internal
+    url: "https://example.org/"
+    interval: 5m
+    conditions:
+      - "[STATUS] == 200"