Add opt to use regex for json path values

[mszostok]

Description

Changes proposed in this pull request:

• Add opt to use regex for json path values

• Docs: Update gh-events source docs botkube-docs#293

Testing

Config:

        repositories:
          - name: mszostok/gh-test-repo
            on:
              events:
                # IssuesEvent with json path filter
                - type: "IssueCommentEvent"
                  # The JSONPath expression to filter events
                  jsonPath: ".comment.html_url"
                  # The value to match in the JSONPath result
                  value: ".*/pull.*"
                  notificationTemplate:
                    previewTpl: |-
                      Commented on #{{ .Issue.Number }} {{ .Issue.Title }}
                      
                      State:   {{ .Issue.State }}
                      Comment: {{ .Comment.Body }}

                    extraButtons:
                      - displayName: Open
                        url: "{{ .Issue.HTMLURL }}"
                        style: primary