Initial support for graph level attributes
Initial support for graph level attributes

Fixes #4 and also fixes writing of graph attributes.

ENCODE-DCC/dagre-d3,cpettitt/dagre,witcxc/dagre,GerHobbelt/dagre,cbfx/dagre-d3,9n/dagre,cpettitt/dagre,git-ashish/dagre-d3,shelsonjava/dagre,shelsonjava/dagre,9n/dagre,leMaik/graphlib-dot,cbfx/dagre-d3,rwl/dagre_charted,9n/dagre-d3,cpettitt/dagre-d3,leMaik/graphlib-dot,tylingsoft/dagre-layout,abduld/dagre,abduld/dagre,git-ashish/dagre-d3,cpettitt/dagre-d3,rwl/dagre_charted,dagrejs/dagre-d3,lmcorbo/dagre,cbfx/dagre-d3,rwl/graphlib_dot,harsha-mudi/dagre-d3,dagrejs/dagre-d3,ENCODE-DCC/dagre-d3,shelsonjava/dagre,witcxc/dagre,rwl/dagre,abduld/dagre,dominiek/dagre-d3,cpettitt/dagre-d3,rwl/dagre_charted,9n/dagre-d3,cpettitt/graphlib-dot,kdawes/dagre,lmcorbo/dagre,dominiek/dagre-d3,harsha-mudi/dagre-d3,dominiek/dagre-d3,kdawes/dagre,9n/dagre-d3,GerHobbelt/dagre,9n/dagre,GerHobbelt/dagre,kdawes/dagre,lmcorbo/dagre,git-ashish/dagre-d3,cpettitt/graphlib-dot,tylingsoft/dagre-layout,cpettitt/dagre