Silence spurious warnings. Add warning on empty block

[preston-evans98]

This commit silences warnings in two cases:

• On the first lang tag of a code block
• When the tag is the tag marking a block for bashtestmd to compile

It adds a new warning when a code block is tagged for bashtestmd but does not include any commands.

$ bashtestmd --input examples/demo-rollup/README.md --output demo-rollup-readme.sh --tag test-ci

Warning: could not find command in block`:
```
export SOV_PROVER_MODE=execute
```
^^^^^ remove the tag test-ci from the block or add a command beginning with `$ ` to fix this warning

[neysofu]

Nice!

[neysofu]

Fixes #1.