Provide autocompletion for environment variables in tox.ini files

[pkirch]

Problem

tox.ini files in larger projects can grow quite large and contain many variables from various sources. In order to ease the development of tox environments a preview feature for environment variables in tox would be helpful.

Solution

• Show previews for environment variables in tox.ini files for passenv and setenv variables.
• Variables could be assigned directly in a section (environment) or in the inherited section testenv which can be reused by all other sections (environments).
• passenv variables are read from the current environment.
• setenv variables are assigned either directly inside the tox.ini file or indirectly referenced from an external .env file.

[The-Compiler]

Implemented in #20