Add support for passing clippy arguments in comment-able file

[gbhand]

Background

As described in monorail#1298, larger-scale CI workflows (often using >100 clippy arguments) are difficult to maintain and understand without the ability to add comments.

Description

Added support for reading clippy args from an sh-style file specified by the args-file parameter. Each individual argument may be on same or different lines. Full-line comments are supported for better organization and clarification. Inline comments are not supported.

Verification

• Success in test run https://github.com/parallelsystems/monorail/runs/7307071120?check_suite_focus=true#step:6:3

[joshparallel]

Looks great! Couple tiny comments.

[joshparallel]

LGTM!