Add a style option to disable point markers on charts

[brianwatling]

With point markers enabled (default): With point markers disabled:

Fixes #594

[dumblob]

Looks good to me in general. My personal preference would be to use a non-negated form (nk_bool show_markers; instead of nk_bool hide_markers;) but I would be fine merging if others will not agree :wink:.

[RobLoach]

That's a good point. Would it be possible to default nk_bool show_markers to nk_true when initializing the struct?

[brianwatling]

That's a good point. Would it be possible to default nk_bool show_markers to nk_true when initializing the struct?

Done - renamed to show_markers and defaults to nk_true to preserve existing default behaviour (and tested that)