Calling new without an argument creates a file called new.yml

## System Info
OS: Mac OS 10.15.1
Muxed Version: v0.7.2-development

## Description
When `muxed new` is called without a `<project>` argument it creates a project file called `new.yml`.

## Steps to reproduce

- Run `muxed new`

#### Expected
Get a message that says an argument needs to be supplied.

#### Actual
A new file `new.yml` is created