Suggestion: make it easier to turn on dangerous mode (or support nims)
Right now, Nimporter offers great low level control over the compiler via `switches.py`. However, as a developer, I don't want to be worrying about maintaining my own copy of `switches.py` for changing simple settings. 

Suppose, after #24, you decide that using the default GC is better. Now, in order to get that update, I can't just update Nimporter; I have to modify my copy-pasted-with-one-added-line `switches.py` file. For the more standard compiler options (danger, checks, opt), I would suggest reading from a config file or taking them as args for `build_nim_extensions`:

```py
ext_modules=nimporter.build_nim_extensions(danger=True, floatChecks=True),
```

Or something like a [`.nims` file](https://nim-lang.org/docs/nims.html#nimscript-as-a-configuration-file):

```yaml
--opt:size
--define:foo
--forceBuild
```

Given that the one of the major use cases for Nimporter is moving computationally intensive tasks to Nim, I imagine many users will want to use `-d:danger` to speed up their pipelines. For me, it makes a 3x difference, so I definitely want to use it.