Use ArgumentParser instead of custom solution

[dannflor]

Currently the implementation of flags isn't conformant to standard convention vapor/docs#689. This and I assume several other issues could be alleviated by using ArgumentParser to format input for a Command.

[0xTim]

Using ArgumentParser is definitely the long term plan. When it was first released it was missing a few features we required to switch over but it might be in a better place now to do so

[gwynne]

#192 begins to address this.