Implement parser and evaluator for TextMate snippets

The Language Server Protocol (LSP) implements TextMate's "snippets"
feature. Originally imagined as a kind of Mad Libs for code, with users
given a code template and some number of "placeholder" blanks to fill
out, which was done by using tab to switch between them, LSP team
attempted to standardize this feature across language servers, so that
language authors had a well-specified interface to provide "snippets" to
any editor implementing the LSP.

Today, the LSP specification of TextMate snippets forms the bedrock of the
ksonnet prototype specification. Specifically, though users will
eventually be able to use Jsonnet to generate prototypes, they are
compiled down to the TextMate snippets specification.

This commit begins this process by introducing an implementation of the
LSP snippets specification, including both a parser and an evaluator.

For more details, see the extensive comment in `interface.go`.