feat(directive): Support for camel casing interpolation variables.

When using a directive with interpolation variables with camel cased
words (camelCase), there was no nice way to specify the values of the
variables with the `translate-value-*` attributes.

You can specify attributes like this `translate-value-first-name="Glenn"`,
which would work together with this translation string `'Hello {{firstName}}'`.
Merge pull request #430 from tregusti/feature/multi-word-values

feat(directive): Support for camel casing interpolation variables.