Made locksmith look at cdk context

[nwouda]

Loads cdk.json from the current working directory and looks for the following context parameters:

"venv": ".venv",
"environments": [
  {
    "account": "1234",
    "name": "acceptance"
  },
  {
    "account": "5678",
    "name": "production"
  }
]

venv should be set to the virtual environment folder. This will cause locksmith to source the bin/activate script on initialising the locksmith session. This is limited to bash, so it will override the LOCKSMITH_SHELL variable.

environments should be a list of accounts and their names. The name is not used by locksmith, but is useful for personal reference or use within the project. Locksmith uses the account numbers to filter out non-related bookmarks.

This change also sets the prompt to make clear what environment is active. This is limited to bash since zsh overrides the passed environment variable.