Support custom size bits or just bits in magic signatures
Hey, I have a usecase (mpeg-ts parser) where there are magic signatures which are arbitrary long bit sized integers. The following usecase currently is not supported:
```
- id: fixed_10
  type: b2
  contents: [2]
```
I thought this could be a workaround (less readable maybe):
```
- id: fixed_10
  contents: [true, false]
```
but it also does not work. It is not a big deal as it is just for confirmation, but would be nice if its supported.