Per @derekrushforth's comments on ActiveCampaign/grunt-postmark#3.

[This](ActiveCampaign/grunt-postmark#3 (comment)) will break things for folks who are already using these changes (@gcoombe and @jmas) as we're now looking for `lowerCamel` variable names instead of `Title` cased ones. Just means a quick find/replace for `Name`/`name, `Subject`/`subject`, etc.

I've internally changed the implementation of `postmark-templates-upload` to support in-memory `htmlBody` and `textBody` as well as lazy loaded paths `htmlSrc` and `textSrc` as @hybernaut originally had it. I didn't actually notice how far my changes had deviated from the default options for the `postmark-templates-upload` task when I filed the PR, but I actually like the idea of loading the template files instead of expecting them to be passed along as task data.

Per ActiveCampaign/grunt-postmark#3.