support dynamically changing properties in template code

## Motivation

If the template has a `public/index.html`, then try to make the following changes:

Update the title of the app to the name of the app. Example:

```html
My App .
```

If the template has a a `public/manifest.json`, modify the `short_name` and `name` fields:

```json
"short_name": "My App",
"name": "My App",
```

## Proposal

The locations where these things need to be changed could be made dynamic using a `.graasprc` file in the template repository. Similar to the way that we delete files listed in a `.graaspignore` file.

Steps:
1. Read a `.graasprc` file in the template repository.
2. This file would have a key like:

```json
{
"replace": [
{
"location": "public/index.html",
"from": "Graasp App Starter",
"to": "{{APP_NAME}}"
},
{
"location": "*.js",
"from": "$MY_ENV_VAR",
"to": "{{APP_SPECIFIC_ENV_VAR}}"
},
]
}
```

As shown above, we could have the replacement strategy access variables set by the user in the CLI. Whether we use `{{` or something else can be also configurable within the same `.graasprc`, to adapt to the different templates.