Tabs with lazy-loaded templates

- [x] create a directive
- [x] create the page with examples
- [x] fix problem with `ag-panel`, see: https://github.com/9ci/rcm/issues/231
- [x] ability to select the first active tab
- [x] support for custom headings `ag-tab-heading`
- [x] ability to select active tab with url params
- [x] support for the context path
- [x] update `ag-plugin` and `ag-demo`