Stuck in "generating markdown" step - potential performance issue

## What did you do
I have 3 json schema files - `A.json`, `B.json`, `C.json`. A references B and B references C. However, when I run the `jsonschema2md` command, command gets stuck at the `gererating markdown` step.

```bash
$ jsonschema2md -d . -e json --out ../out
loading 3 schemas
writing schemas
writing schemas to ...
writing README
building readme
writing documentation
generating markdown // stuck here
```
In my research so far, it seems like this happens when there are too many incoming references to a definition and/or when an `array` based reference references another `array` based reference.

Please see the example below of all 3 files, I deliberately added few extra properties `extends[n]` to the `A.json` schema, which made the command got stuck. Please help.

## What did you expect to happen
Markdown to generate successfully.

## What happened
Command got stuck at `generating markdown` step

## What's your environment

* Operating System: macOs High Sierra `10.13.6`
* node.js version: `v14.3.0`

## Do you have example files:
`A.json`
```
{
"$id": "https://foo.bar/A",
"$schema": "http://json-schema.org/draft-07/schema#",

"type": "object",

"properties": {
"extends": {
"type": "array",
"items": {
"$ref": "https://foo.bar/B"
}
},
"extends2": {
"type": "array",
"items": {
"$ref": "https://foo.bar/B"
}
},
"extends3": {
"type": "array",
"items": {
"$ref": "https://foo.bar/B"
}
},
"extends4": {
"type": "array",
"items": {
"$ref": "https://foo.bar/B"
}
},
"extends5": {
"type": "array",
"items": {
"$ref": "https://foo.bar/B"
}
}
}
}
```
`B.json`
```
{
"$id": "https://foo.bar/B",
"$schema": "http://json-schema.org/draft-07/schema#",

"type": "array",
"items": {
"$ref": "https://foo.bar/C"
}
}
```

`C.json`
```
{
"$id": "https://foo.bar/C",
"$schema": "http://json-schema.org/draft-07/schema#",

"type": "string"
}
```

Reproduce the hang with the jsonschema2md command and the provided A.json, B.json, and C.json files, focusing on the generating markdown step. Trace that entry point to determine why repeated array references prevent completion. Done means the example schemas generate Markdown successfully without remaining stuck.

Written by the indexing model from the issue text.