Improve config checking: ensure mustache variables exist

[andykais]

Using this issue as a guide: janl/mustache.js#538 it is possible to extract the variable names for a template. Therefore, I should be able to check if a given template is using valid variable names. E.g.

// check against builtin variables
"https://example.com/post/{value}/comments/{index}" // valid

// given { "input": ["search", "user"] }
"https://example.com/user/{user}/search/{search}" // valid
"https://example.com/user/{user}/search/{someotherval}" // invalid

[andykais]

also useful: handlebars-lang/handlebars.js#1207. Consider using the barhandles repo