Add ability to show custom dialogs

**Is your feature request related to a problem? Please describe.**
Instead of the simple `prompt`-Dialog, I want to show my own dialog at least for links and images.

**Describe the solution you'd like**
The option `promptTexts` could be extended to allow a function, too.
If you treat the functions return value as promise, the solution could be easily done with something like

```js
function drawLink(editor) {
var cm = editor.codemirror;
var stat = getState(cm);
var options = editor.options;
var url = 'https://';
var text = options.insertTexts.link;
var promptResult;

if (options.promptURLs) {
var promptText = options.promptTextslink;
if (typeof promptText === 'function') {
promptResult = promptOption(type);
} else {
promptResult = prompt(promptOption, url);
}
} else {
promptResult = url;
}

Promise.resolve(promptResult)
.then(function (url) {
drawLinkFinally(editor, url)
})
.catch(function () {});
}

function drawLinkFinal(editor, url) {
var cm = editor.codemirror;
var stat = getState(cm);
var options = editor.options;

if (!url) {
return false;
}

url = escapePromptURL(url);
_replaceSelection(cm, stat.link, options.insertTexts.link, url);
}
```

If you don't want to depend upon `Promise`, you can detect if there's a `then`-function on the result and use it that way, or (if the `then` function is missing) use the `drawLinkOrImageFinal` directly.

**Additional context**
It would be great, if the `_replaceSelection` would not only replace `#url#` from a string, but all object values of a map.
If you e.g. use `{ url: "https://xyz", linkText: "My URL" }` instead of a string, and the insertTexts-pattern is `[#linkText#](#url#)`, you can have much more complex dialogs.
`escapePromptURL` must be aware in that case, too..

Start by tracing the link and image dialog flow around promptTexts, drawLink, and drawLinkFinally. Inspect _replaceSelection and escapePromptURL to understand the existing string-only behavior. Done means custom synchronous or promise-based dialogs work for links and images, and object values can populate insertTexts patterns without breaking URL escaping.

Written by the indexing model from the issue text.