Allow defining a template for how paths should be passed to the command

At this moment lint-staged will always pass file paths as arguemnts at the end of the string:

npm run-script command -- path/to/staged/file1 path/to/staged/file2
eslint -- path/to/staged/file1 path/to/staged/file2

In order to support exotic cases like #136 and #18 we might want to redefine this. One way of doing this would be to pass a “template” string to how paths should be interpolated. Something like this:

{
  "*.jpg": "${ path } | imagemin",
  "*.html": "gulp my-task -g ${ paths }",
  "*.js": "eslint" // This is the current way of doing "eslint ${ paths }"
}

lint-staged will try to interpolate those strings when running tasks.

1. The piping example will require calling filepath | imagemin for each path in paths. Not sure how to do that. Different type of token (path instead of paths?)
2. Better error handling: since templates may be broken, lint-staged should help users find the mistake

Another use cases: #404

I’m open for a discussion and suggestions on the API of this feature.

[renchap]

Small comment here about usecase: being able to do something like: *.ts: "tsc --noEmit -p tsconfig.json"

would be quite useful to run the Typescript compiler in check mode only if there are staged Typescript files. This is different from running it using Husky, as you only want to check it if you committed TS files, not all the time.

[okonet]

Another important question if implicit syntax { "*.js": "eslint" } should be deprecated in favor of more explicit one i.e. {"*.js": "eslint <paths>" }