Click-odoo

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- pass click ctx to environment_manager

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- refactor ``click_odoo.env_options`` to allow propagating ``env``
to subcommands through the click context. Fixes 16.
- do not log click Exit exception, which may be raised by subcommands as
a result of --help.

Upgrading:
- click-odoo now requires click>=7

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- add ``with_addons_path`` option to ``click_odoo.env_options``
to control the presence of the ``--addons-path`` option. Defaults to False.
Enabled for the ``click-odoo`` CLI.

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- add ``environment_manager`` to ``click_odoo.env_options``, providing
a hook on ``odoo.api.Environment`` creation.
- add ``--addons-path`` option to the CLI.
- add ``database_must_exist`` option to ``click_odoo.env_options``
so scripts can behave how they please in case the database is absent.

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- silence deprecation warning
- adapt tests for Odoo 12

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- clarify the behaviour of ``env_option`` ``with_database`` and ``database_required``
parameters; in particular, when ``with_database`` and ``database_required``
are both set (the default), the ``--database`` option can be omitted
as long as a database is declared in the Odoo configuration file.