Defer the implementation of select functions in the tech to source handlers if they provide them

[imbcmdth]

I wrote this to allow us to generically add "deferrable" functions in the tech. Right now we only need two but we could extend that later.

[pam]

Tests failed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: BUSTED

Commit: 4b67a05
Build details: https://travis-ci.org/pam/video.js/builds/88241048

(Please note that this is a fully automated comment.)

[pam]

Tests passed. Automated cross-browser testing via Sauce Labs and Travis CI shows that the JavaScript changes in this pull request are: CONFIRMED

Commit: b7b0c12
Build details: https://travis-ci.org/pam/video.js/builds/88397726

(Please note that this is a fully automated comment.)

[gkatsev]

So, this basically just makes the seekable and duration functions into a generic function that delegates to the source handler?

[imbcmdth]

Yes. Seekable was already had a delegate but I felt that making it so that delegates can be added easily without replicating code was worth the effort.

[gkatsev]

LGTM

[gkatsev]

Also, yay, travis is finally reporting back again