read from `--stdin`

### Describe the desired feature

Useful for dev tooling.

Possible APIs
```
cat src/Contract.sol | slither - --stdin-filename src/Contract.sol
cat src/Contract.sol | slither - --base-path src/
```

`solc` API is like:
```
cat src/Contract.sol | solc - --allow-paths "*," --base-path ./src
```
`--base-path` is necessary for import resolution. Probably `--stdin-filename` (following language tooling conventions) would be better, as it provides more information.