chore(destroy): add current path . as the default path

[Triple-Z]

Signed-off-by: Triple-Z me@triplez.cn

closed #420

[Triple-Z]

This PR will have the following outputs.

$ ~/Code/envd/bin/envd destroy --help
NAME:
   envd destroy - destroys the envd environment

USAGE:
   envd destroy [command options] [arguments...]

OPTIONS:
   --name value, -n value  Name of the environment
   --path value, -p value  Path to the directory containing the build.envd (default: current directory)

BTW, maybe down is a better word than destroy:

1. docker-compose uses it.
2. down is the antonym for up.
3. destroy is easy to typo :(

cc @gaocegege

[gaocegege]

Thanks for your contribution! 🎉 👍

[gaocegege]

@all-contributors Please add @Triple-Z for his code contributions!

[allcontributors]

@gaocegege

I've put up a pull request to add @Triple-Z! 🎉