Make FPN, top-down and backbone configurable

[julienr]

This adds two new config options : Config.FPN_CLASSIF_FC_LAYERS_SIZE and Config.TOP_DOWN_PYRAMID_SIZE to allow the user to specify the size of those layers.

This also adds a Config.BACKBONE option that allow one to use a custom backbone by providing a function that should return a backbone graph.

Also fix #170 and allow dashes in model name when trying to restart from an epoch.

[benqua]

+1 would love to see this PR merged. It fixes two issues I am facing right now!

(BTW, wouldn't it be better (easier to get merged) to have 4 distinct PR?)

[julienr]

I can definitely split this into 4 if it makes things easier; whatever the maintainer prefers

[waleedka]

Interesting additions. The three commits that add the backbone configuration and allow dashes in model names look good and elegantly done.

The fix for #170 is, I think, a step in the right direction but doesn't completely solve the problem. It solves the case when there are no ground truth boxes, but the code still fails at a later stage. In my test, on TF 1.8, I get a core dump. But, one step at a time, so I'll merge this and address the other issue separately.

Thank you, great PR.

[julienr]

Thanks for merging !