Apply indentation not only for the first line

[zizo007]

When using mdx_include with other mkdocs extensions such as admonition it is important to apply the indentation for all the lines being included.

For example:

Suppose we have an md we would like to include:

My included content.
Which can have multiple
lines.

And blank lines
as well.

!!! note "This is only a test"
    {! included_item !}

Currently, this results in the following:

!!! note "This is only a test"
    My included content.
Which can have multiple
lines.

And blank lines
as well.

which is not suitable for this scenario. Instead it should generate this:

!!! note "This is only a test"
    My included content.
    Which can have multiple
    lines.
    
    And blank lines
    as well.

How about a syntax enhancement for this feature?
I would gladly develop this feature.

[neurobin]

How about a syntax enhancement for this feature?

Introducing a new syntax feature, well, that sounds good.

[zizo007]

How about this changeset?
Please review and address if something is missing.