Add support for fixing indentation in jinja

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[timvink]

@fralau mkdocs-macros-plugin will not keep the indentation if you insert multiple lines. This is a problem for example when inserting a table into a content tab.

I've fixed this by adding a custom filter to macros. See the PR, should be clear.

Perhaps you have a better solution, solving this natively inside the macros plugin?
If not, you might want to copy the add_indentation filter to macros by default and document it's usage with an example in the macros docs.