Additional function about get the start vertex from the file

[lxrzlyr]

modified：     src/bfs.cc
modified：     src/command_line.h
modified：     src/benchmark.h

[sbeamer]

As I discussed in #40, I will not accept this functionality into the code base for fear it might be abused leading to poor evaluations.

If this functionality were to be accepted, there are some ways to improve this pull request (PR). Please keep an eye on the differences it introduces, and make sure they are all necessary. In this case, it touches 874 lines, when the critical changes are only a few dozen. Most of those changes appear to be changes to the coding style and format. Those are not necessary to submit the change, and this repo mostly conforms to Google's C++ Style Guide. Additionally, one of those changes even disables all the kernels other than BFS. That would not be good for us to include by default.