Add ability to inject part of the code

Example, if I want to include this fragment https://github.com/gajus/gitdown/blob/12c91a46f33a0c926a1baed381d0636968c4f0f9/src/gitdown.js#L27-L50

I would surround the code fragment with comments such as:

```js
// gitdown-template-start: foo-fragment
/**
* Process template.
*
* @returns {Promise}
*/
gitdown.get = () => {
return parser
.play(input)
.then((state) => {
let markdown;

markdown = state.markdown;

if (gitdown.getConfig().headingNesting.enabled) {
markdown = Gitdown.nestHeadingIds(markdown);
}

return gitdown
.resolveURLs(markdown)
.then(() => {
return markdown.replace(//g, '');
});
});
};
// gitdown-template-end: foo-fragment

```

where "foo-fragment" is name of the fragment.

Then this snippet can be included into a README document using:

```js
{"gitdown": "snippet", "file": "./src/gitdown.js", "name": "foo-fragment"}

```

This would be used to inject type declarations, etc. to documentation without needing to repeat them.