Make credits configurable on Stanford CME

* Assumes cert_data stanza now has additional "CREDITS" subdict, with a
  key "COUNT" and a key "TYPE". These are interpolated directly onto the
  certificate.

@stvstnfrd, please. @caesar2164, fyi.

My hope, @sarina, is not only that this will help us, but that it will make the cme template marginally more useful to members of the open source community.... (But firstly it helps us by letting us support courses worth lots of different credit values.)
Nit: I'm wondering if it makes sense to just have this be a single
string which is tacked on if set (combining `credit_count` and
`credit_type`). So in your config YAML, you'd have
```
Course/Key/Triple:
    CREDITS: "23.5 <i>AMA PRA Category 1 Credits</i><super><font size=13>TM.</font></super>
```
Given that this is already basically a hard-coded display string, it's
unclear to me what we gain by having two separate variables (besides the
extra lines of code).