Added configurable spacing after and before headers

[MayitaMayoso]

I felt the spacing after headers was too big so I added the ability to configure how big are the spaces after and before the headers.

[MayitaMayoso]

IDE keep messing with the format, please ignore my previous commits to my fork 😅.

[dougbinks]

Thanks for this PR.

Your implementation as it stands will break current users layout, so it needs to be changed so that the default is to use ImGui::NewLine(). I think the easiest way to do this is to use a negative spaceBefore or spaceAfter to mean use Newline() and to default to -1.0f in the struct MarkdownConfig:

    MarkdownHeadingFormat   headingFormats[ NUMHEADINGS ] = { { NULL, true, -1.0f, -1.0f }, { NULL, true, -1.0f, -1.0f  }, { NULL, true, -1.0f, -1.0f  } };

Note you also need to remove the defaults from struct MarkdownHeadingFormat as these should be set as above (this makes it easier for people to see how to initialize these).

[MayitaMayoso]

Added the mentioned suggestions! I did set as -1.0f the default values so it does not affect the format of other people.

[dougbinks]

Thanks - I'll pull this to review once I get some time.

[dougbinks]

This does not compile for me as the function ImGui::Spacing() does not have a parameter:

https://github.com/ocornut/imgui/blob/master/imgui.h#L482

I think you could possibly use a Dummy of a given height and zero width?

[dougbinks]

Just a reminder that this PR is waiting on a fix as it does not compile against the latest Dear ImGui library code.