qca: add options for head, qt5, tests, api docs and building all plugins

@mikemcquaid Do you think all of the plugins should just be `:recommended` (excepting `qca-ossl` which should always be built), since they all successfully build?
This appears to not be the case (at least not consistently) when building using the current formula (not this PR's):

```
$ brew install --build-from-source qca -v
...
Plugins:
  qca-botan on  <-- should be off
  qca-cyrus-sasl on  -- Not Applicable
  qca-gcrypt on  <-- should be off
  qca-gnupg on  <-- should be off
  qca-logger on  -- N/A
  qca-nss off  <-- correct
  qca-ossl on  -- N/A
  qca-pkcs11 off  <-- correct
  qca-softstore on  -- N/A
...
```

It looks like QCA's CMake find modules are looking in `/usr/local/*` regardless of what `superenv` is offering. That's pretty common, though. If I was installing in a custom HOMEBREW_PREFIX or the dependent formulae were all keg-only, then I would expect to see what you are referring to.