Register 'commonmark/hard_break' as an custom option

So we have all the default parameters in one place,
and not hidden within the renderer function.
Also adapt the 'getOption' function such that it

Register 'commonmark/hard_break' as an custom option

So we have all the default parameters in one place,
and not hidden within the renderer function.
Also adapt the 'getOption' function such that it is able
to retrieve both, leage-commonmark options as well as our
custom registered commonmark options.

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Merge branch 'hardbreak'

Use CoreInlineElement for Newline

first 2.0 implementation - at least no error

theleague/commonmark 2.0.x compatibility matching

fix filename case