Loading graphs as a set of Matrix-Market files

[georgiy-belyanin]

We already have some consistent representation of graphs as sets of adjacency matrices in Matrix-Market format. We also have a script for converting N-triples into such format. It can be described as follows:

- '<output-dir>/<number>.txt' is an adjacency matrix in Matrix-Market format
  for the predicate <number>
- '<output-dir>/vertices.txt' is a map from vertices to its number
- '<output-dir>/edges.txt' is a map from edges to its number

Let's support it by default graph format and let pathrex CLI be started as ./pathrex <path-to-dir-with-mm-files>.