Add support for configurable warnings

New CLI option --warn-conf accepts a comma-separated sequence of
<filter>:<action> pairs for customizing warnings.

Supported filters are:
* any          - matches all warnings
* id=<integer> - matches warnings with the integer id
* src=<glob>   - matches warnings where <glob> matches the warning
                 source file

id and src filters can be combined with &.

Supported actions are:
* :s - suppress the warning
* :w - report the warning as a warning (default behavior)
* :e - error on the warning

For example, the new Vec dynamic width matching warning can be
suppressed for source files in src/main/scala while all other warnings
elevated to errors with:
  --warn-conf "id=3&src=src/main/scala/**:s,any:e"