Webfont did not receive a popper JSON result

grunt-webfont crashed on my machine with a fatal error:
```
grunt
Running "webfont:icons" (webfont) task
Fatal error: Unexpected token P
```

further investigations  showed that the JSON value had a prefix (beginning with a P):
```
PythonUI_Init()
copyUIMethodsToBaseTable()
{"names": ["icon-social-twitter"], "file": "build/fonts/icons-f65c6f7c13c8dfaac01df545dcb55578"}
```

This pull request tries to remove prefixes and postfixes which fixes my problem.

**Bonus** - if there will be a similar bug in future the code would output a more verbose error message:
```
grunt
Running "webfont:icons" (webfont) task
Warning: Webfont did not receive a popper JSON result.
SyntaxError: Unexpected token P 
PythonUI_Init()
copyUIMethodsToBaseTable()
{"names": ["icon-social-twitter"], "file": "build/fonts/icons-f65c6f7c13c8dfaac01df545dcb55578"} Use --force to continue.
```