Feature Request: section headers and footers support multiple lines

Currently, I use region folding to nest away and hide my imports:

################################################################################
#region Imports

#===============================================================================
#region stdlib

import sys

#endregion stdlib
#===============================================================================

#===============================================================================
#region third party

import pytest

#endregion third party
#===============================================================================

#endregion Imports
################################################################################

such that they fold into:

################################################################################
#region Imports

#===============================================================================
#region stdlib
#===============================================================================

#===============================================================================
#region third party
#===============================================================================

#endregion Imports
################################################################################

and

################################################################################
#region Imports
################################################################################

It would be really awesome if isort supported multi-line section headers and footers in order to support both fixing and understanding the existence of headers/footers like these.

For config, it would be great if both of these formats were handled (I prefer the array version, but others might not):

import_heading_stdlib = "#===============================================================================\n#region Standard Library\n"

or

import_heading_stdlib = [ 
    "#===============================================================================",
    "#region Standard Library",
    ""
]

Currently if the single string style is used, headers and footers just get repeated.
Currently if the array style is used, all text between and including the square brackets are joined as a string and used.

Start by tracing isort's existing handling of import_heading configuration, including the single-string and array forms shown in the issue. Verify how headers and footers are currently parsed and emitted; done means both formats preserve multiple lines without repeating headers or joining array syntax incorrectly.