Add the [<ignore>] syntax

[sanex3339]

For our case, we need the support of the [<ignore>] syntax.

const template = () => {
  return (
    // [<snippet example>]
    <Provider
      // [<ignore>]
      propToIgnore={propToIgnore}
      // [<endignore>]
      component={() => <div>Data is loading...</div>}
      errorComponent={({ message }) => <div>There was an error: {message}</div>}
    >
       <div>...</div>
    </Provider>
    // [<endsnippet example>]
  );
};

So the result is:

    <Provider
      component={() => <div>Data is loading...</div>}
      errorComponent={({ message }) => <div>There was an error: {message}</div>}
    >
       <div>...</div>
    </Provider>

So, what we want is to exclude a subset of an imported snippet.
Also I added a few tests

Fixes #14

[sanex3339]

@ilya-lesikov hi, would you mind taking a look please