Add support for custom content in the client's configuration file

[ohmer1]

I added support for custom content in the client's configuration file. My use case was to be able to add a custom include to a client without having to define a new profile just for this addition. To use that feature, just add a content key when defining a client in burp2_add_manual_clients. The content key is of course optional, so this shouldn't break anything.

Example:

burp2_add_manual_clients:
  - name: client_name
    profile: profile name to use (optional), default: profile_lnxsrv (these files are in incexc/ dir).
    password: client_password (optional), default: burp_client_password var
    content: 
      - 'include = E:/'
  - name: second_client

[pablodav]

Thanks you!
This looks very good, I have some issues with the automatic tests that I need to troubleshoot before merging.