[sqllab] add support for Jinja templating (#1426)

* [sqllab] add support for Jinja templating

* Adressing comments

* Presto macros

* Progress

* Addressing coments

docs/sqllab.rst

@@ -0,0 +1,57 @@
+SQL Lab
+=======
+
+SQL Lab is a modern, feature-rich SQL IDE written in
+`React <https://facebook.github.io/react/>`_.
+
+
+Feature Overview
+----------------
+- Connects to just about any database backend
+- A multi-tab environment to work on multiple queries at a time
+- A smooth flow to visualize your query results using Caravel's rich
+  visualization capabilities
+- Browse database metadata: tables, columns, indexes, partitions
+- Support for long-running queries
+  - uses the `Celery distributed queue <http://www.python.org/>`_
+    to dispatch query handling to workers
+  - supports defining a "results backend" to persist query results
+- A search engine to find queries executed in the past
+- Supports templating using the
+  `Jinja templating language <http://jinja.pocoo.org/docs/dev/>`_
+  which allows for using macros in your SQL code
+
+
+Templating with Jinja
+---------------------
+
+.. code-block:: sql
+
+    SELECT *
+    FROM some_table
+    WHERE partition_key = '{{ preto.latest_partition('some_table') }}'
+
+Templating unleashes the power and capabilities of a
+programming language within your SQL code.
+
+Templates can also be used to write generic queries that are
+parameterized so they can be re-used easily.
+
+
+Available macros
+''''''''''''''''
+
+We expose certain modules from Python's standard library in
+Caravel's Jinja context:
+- ``time``: ``time``
+- ``datetime``: ``datetime.datetime``
+- ``uuid``: ``uuid``
+- ``random``: ``random``
+- ``relativedelta``: ``dateutil.relativedelta.relativedelta``
+- more to come!
+
+`Jinja's builtin filters <http://jinja.pocoo.org/docs/dev/templates/>`_ can be also be applied where needed.
+
+
+.. autoclass:: caravel.jinja_context.PrestoContext
+    :members: