More api changes

[Zulko]

Applies these changes:

clip.with_subclip(start, end) -> clip.subclipped(start, end)
clip.with_multiply_volume -> with_volume_scaled(factor)
clip.with_multiply_speed -> with_speed_scaled(factor)
clip.with_cutout(start, end) -> with_section_cut_out(start, end)
clip.with_on_color(color) -> with_background_color(color)
clip.with_add_mask -> with_mask (I'll add a default full-1s mask)
clip.with_make_frame -> with_updated_frame_function
clip.with_sub_effects -> with_effects_on_subclip(effects, start, end)
clip.with_layer -> with_layer_index

[coveralls]

coverage: 86.428%. remained the same
when pulling aaf2985 on zulko/api-changes
into c88852f on master.