Switch to a single key for command and command shell

This makes the configuration more similar to Minitor-py and
docker-compose. If a string is passed, it will be executed in a shell.
If an array is passed, it will be executed in as a command directly.

This breaks compatiblity with previous versions of Minitor-go, but
closer to compatiblity with Minitor-py.

test/valid-verify-multi-line.yml

@@ -3,7 +3,7 @@ check_interval: 1
 
 monitors:
   - name: Shell
-    command_shell: >
+    command: >
       echo 'Some string with stuff';
       echo "<angle brackets>";
       exit 1
@@ -13,6 +13,6 @@ monitors:
 
 alerts:
   log_shell:
-    command_shell: |
+    command: |
       echo 'Some string with stuff'
       echo '<angle brackets>'