Print graphs in gml format

GML is a widespread format for printing graphs, which can be used later by
graph visualization tools. Here, we add support for this format via the "-g"
option in the CLI.
Don't print the progressbar when the output is redirected

EAST currently prints some progressbar-like strings while calculating
keyphrase tables & graphs using caret returns ('\r'). That's not OK,
however, when the output is redirected to a file. Here, we add checks
for that, so that the progressbar is printed only in the CLI.