We should embed template files for ease of installation

Previously we mentioned in #66 (closed) and #109 (closed) that we can't use cargo install and at the time we figured that installation via deb or similar was preferable. However as we work toward being able to use subplot to build rust test suites we need to consider how the Rust community tends to install tools -- namely cargo install.

A step toward making that possible is to ensure we embed templates and referenced files during cargo build so that we can find them even if they're not on the filesystem.

To consider this issue done, the following is needed

• Embed template metadata and associated files a build time
• Support finding templates either embedded or on the filesystem
• Support overriding the embedded content with files on the filesystem