More quiet output by default; only emit warnings if we have something to say

[neysofu]

Currently, the sovereign-sdk-wip demo-rollup READMEs emit this output:

Unknown bashtestmd tag, ignoring: shell
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: sh
Unknown bashtestmd tag, ignoring: test-ci
Unknown bashtestmd tag, ignoring: bash
Unknown bashtestmd tag, ignoring: test-ci

It's not immediately clear whether one should worry about this. The solution is to not warnings for:

• bash
• shell / sh
• The value of only_tag

[neysofu]

Fixed by #3.