Add Atlassian Bitbucket Pipelines build system support

[devlead]

Atlassian's retiring their Bamboo Cloud offering replacing with Bitbucket Pipelines and would be nice if Cake supported it.

I'm already on the closed beta testing this, so assigning issue to me.

Documentation here

Environment variables of interest:

Default variable	Description
BITBUCKET_COMMIT	The commit hash of a commit that kicked off the build
BITBUCKET_BRANCH	The branch on which the build was kicked off. This value is only available on branches.
BITBUCKET_TAG	The tag of a commit that kicked off the build. This value is only available on tags.
BITBUCKET_REPO_SLUG	The URL-friendly version of a repository name. For more information, see What is a Slug.
BITBUCKET_REPO_OWNER	The name of the account in which the repository lives

Note: BITBUCKET_BRANCH and BITBUCKET_TAG are mutually exclusive. If you use both, only one will have a value.