API: Support inequalities in templates, fixes #1518

This adds support for `<`, `<=`, `>`, `>=` in templates (see #1518 for original ticket).

This required a change to the FreeString, as this now has to be bounded by a `<` or a `>`-sign, otherwise for the following example the free string would be `5 >`
```
<% if 5 > 3 %>...<% end_if %>
```

This includes some test cases for all inequalities and their equal-counterparts as well as a little doc-item about these operators...

Best regards,
s-m