Default to `run` command if not specified

You can now run venus without specifying the `run` command. This means that:

`venus run -t test1.js,test2.js -e ghost`

is now equivalent to:

`venus test1.js test2.js -e ghost`

and:

`venus test1.js,test2.js -e ghost`

Additionally, running `venus` with no arguments is now equivalent to running `venus run -t .`.