autosummary: Allow customizing template environment

[flying-sheep]

the autosummary extension uses this code to create a jinja template environment.

there is no room for customization via conf.py, and users will have to modify e.g. jinja2.defaults.DEFAULT_FILTERS if they want to add filters for autosummary pages.

it would be useful to have customization options like html_context, e.g. for usage like this: readthedocs/readthedocs-sphinx-ext#28

[tk0miya]

+1: Reasonable.

[tk0miya]

@flying-sheep Do you still need such a feature? If true, I'll add autosummary_context to add template variables for templates of autosummary.

[flying-sheep]

Yes, It would be cleaner to do it that way for sure!

[tk0miya]

Okay, I'll post a PR later.

[flying-sheep]

Great, thank you!