Environment variables fail when not strings

[Cybolic]

When giving a numerical environment variable in a config, such as SERVER_PORT: 1999, tmuxp will break with a TypeError exception but provide no more information as to why.

The issue is in config.py line 216:

val = expandshell(val)

I believe this line should either be wrapped in a try and provide an error message telling which variable didn't work or just cast all environment variables to strings (as in val = expandshell(str(val))). Whether it should refuse to continue or just ignore the error I'll leave up to the maintainer.

Example config that fails:

session_name: Example
environment:
  SERVER_PORT: 1999
windows:
  - window_name: something
    panes:
      - shell_command: echo "server port: ${SERVER_PORT}"