A more fool-proof frontend parser selection scheme

[dboyliao]

Current frontend parser is selected via given graph file extension (such as .pb for an GraphDefParser...etc)
@mbartling and @neil-tan are talking about a more fool-proof scheme at PR #49.
Planning to implement it in the future PRs

[Michael Bartling]

@dboyliao Can we close the issue as #49 has been implemented? Or should it remain open?