Simplify panes and display github markdown flavor in human-pane

[bourgeoa]

The actual markdown-pane do not display the full markdown flavor.
A panes simplification can be achieved displaying markdown directly in human-pane

[bourgeoa]

displayed with Human-pane and PR #316
internal links are working -
edit made only via source-pane

displayed with actual markdown-pane
html code in markdown displayed as text
edit made in markdown-pane and in source-pane

[bourgeoa]

@angelo-v PR #316