Add ability to set source folder in flow dev

[chasefleming]

If you are running flow dev, you need to have a cadence folder at the root. Not all projects are structured like this. For example, in many frontend apps, code is in src/ and a cadence folder would most likely be in src/cadence.

It would be nice if you could set the source folder so flow dev is more flexible. Maybe a command like:

flow dev --src ./src