PopBubbles: Add options for output graph format

Rename --dot to --bubble-graph
Overlap: Add options for output graph format
MergePaths: Read contig lengths from stdin
PathConsensus: Add options for output graph format
abyss-pe: Change the graph format to GraphViz
Graph/GfaIO.h: Bug fix. Ignore quality.
Graph/GfaIO.h: Read the coverage property XC
AdjList: Add options for output graph format
FilterGraph: Add options for output graph format
Add --gv as a synonym for --dot
abyss-pe: New parameter to select graph format

Conflicts:
	bin/abyss-pe
PathOverlap: Add options for output graph format