Don't store file content in memory

[allejo]

Whenever the file is read in as a FrontMatterDocument, the file's contents are stored in memory as $this->bodyContent. The more files that are parsed, the more that is being stored.

I want to explore changing this behavior to reading in the body of the files only when the DevServer needs it; the FrontMatter will still be read in and stored but the body will be thrown away until it's needed again.

For a regular build, however, the Compiler should store things in memory for faster performance.

[allejo]

Since this is likely going to be a very invasive change with how stakx core works and we're in a feature freeze for 0.2.0. I'll move this to the next major release.