feat: better CLI configuration setup

[mathnogueira]

This PR adds a command tracetest configure that allows you to define the configuration of the CLI. Now you don't have to manage the config.yml file manually.

Other than that, it also supports a global configuration inside ~/.tracetest/config.yml. This is a fallback configuration that is only used if the current directory doesn't contain any config file in it. You also can write the configuration in the global config file by executing tracetest configure --global.

$ tracetest configure

# (current value: <value>) is only displayed if there is a config.yml file in the directory and it is 
# not empty. It shows the current value of that variable
Enter your Tracetest server URL (current value: http://localhost:8080): 

$ tracetest configure --global

# same thing as above, but the file ~/.tracetest/config.yml must exist for the current
# value be shown to the user
Enter your Tracetest server URL (current value: http://localhost:1234): 

Checklist

• tested locally
• added new dependencies
• updated the docs
• added a test

[schoren]

Awesome feature!