Add support for superseding runs (#831)

closes #11

Added support:
1. Environment variable `WOODPECKER_DELETE_MULTIPLE_RUNS_ON_EVENTS` (Default pull_request, push)
2. Builds will be marked as killed when they "override" another build

docs/docs/20-usage/71-project-settings.md

@@ -40,3 +40,6 @@ You can change the visibility of your project by this setting. If a user has acc
 
 After this timeout a pipeline has to finish or will be treated as timed out.
 
+## Cancel previous pipelines
+
+By enabling this option for a pipeline event previous pipelines of the same event and context will be canceled before starting the newly triggered one.

docs/docs/30-administration/10-server-config.md

@@ -197,6 +197,11 @@ Link to documentation in the UI.
 
 Always use authentication to clone repositories even if they are public. Needed if the forge requires to always authenticate as used by many companies.
 
+### `WOODPECKER_DEFAULT_CANCEL_PREVIOUS_PIPELINE_EVENTS`
+> Default: `pull_request, push`
+
+List of event names that will be canceled when a new pipeline for the same context (tag, branch) is created.
+
 ### `WOODPECKER_DEFAULT_CLONE_IMAGE`
 > Default: `woodpeckerci/plugin-git:latest`