Support custom tags in EGL

It would be useful to allow users to change the default EGL tags, like [Jinja does](https://jinja.palletsprojects.com/en/3.1.x/api/). For example, users could change the start/end dynamic section tags from `[%` and `%]` to `<%` and `%>` and write EGL templates like this:

```
<%for (i in 1.to(2)){%>
Number <%=i%>
<%}%>
```