Fix bug in how args are parsed from site-config

[jlashner]

Small change to fix the bug kyohei found here: #377
Turns out the ArgContainer site-config uses to parse site-config args was treating negative numbers like new arguments, which was causing some issues.

Description

ArgContainer now checks that the first char after leading hyphens is in the alphabet, when determining if it should create a new arg entry.

Motivation and Context

#377

How Has This Been Tested?

Tested locally that new namespace looks like I'd expect

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.