Only plain literals should be parsed

@dbieber correctly pointed out in #95 that there is a more general problem with the parsing code. This is a follow up issue to that issue which only dealt with the most common case of a single argument.

Here are some cases demonstrating
```bash
$ python3 -c "import fire.parser as p; v = p.DefaultParseValue('[1+1]'); print(type(v), v)"
[2]
$ python3 -c "import fire.parser as p; v = p.DefaultParseValue('{key: 1+1}'); print(type(v), v)"
{'key': 2}
```

Some options are:
1. Accept the existing behaviour.
2. Always parse the whole argument as a string by doing `raise ValueError` in ``fire.parser._LiteralEval`
3. Parse only the sub expressions as strings

For 2. and 3. I think these would be the valid types from the `ast` module.

- `Str`
- `Name` (converted to `Str`)
- `Num`
- `List`
- `Tuple`
- `Dict`
- `Set`
- `UnaryOp`