Support sourced file names in tmux format

[Frederick888]

Recently oh-my-tmux [1] updated to support placing its files in
directories other than HOME like ~/.config and XDG_CONFIG_HOME.

So now instead of a static file name, we need some sort of variable
expansion and I think tmux format fits the role quite well.

[1] https://github.com/gpakosz/.tmux

[Frederick888]

I don't have Vagrant and I'm not sure where I can find the ./run_tests file.

If someone can help check if the new test is actually valid that'd be great 🙏

[gpakosz]

I don't think this is needed. And this won't help Oh my tmux! which specifically now relies on setting @tpm_plugins before invoking TPM

[gpakosz]

I don't think this is needed

Well maybe it helps in other situations but for Oh ny tmux! I reverted to filling @tpm_plugins instead of relying on TPM to discover plugins. In order not to impose Oh my tmux!'s opinions on TPM

[Frederick888]

@gpakosz found a solution for the original issue so I personally don't need this PR any more.

So if the maintainers don't find it particularly useful either, please feel free to close it :)