Revert "Fixed #16. Fixes yamlString, yamlScalar."

This reverts commit 8050b3fb42a35ff66b0616c8d85653696ed686b5.
Documentation: suggest using modelines.

After PR #19, ansible is a filetype, so it can be set from a modeline.
Issue #2: Add the jinjaNestedElement syntax cluster.

Add this cluster, then replace a bunch of
containedin=jinjaVarBlock,jinjaNested clauses with
contains=@jinjaNestedElement on the jinjaVarBlock and jinjaNested syntax
groups.
Issue #2: Add ansibleKey and ansibleVarBlock syntax items.

These replace the jinja variants for mappings where the key is one of
the Ansible keywords when, changed_when, or with_*. The ansibleVarBlock
item can contain exactly the same things as the jinjaVarBlock element,
namely the @jinjaNestedElement syntax cluster.