line_length: Extract inline logic to new config option

This commit extracts the inline mappings logic defined in the previous
commit to a separate config option, as suggested by @adrienverge. I'll
squash this into the previous commit if the change is accepted. (I named
the option slightly differently to what was suggested as I think my
proposal reads better without consulting the docs: I'd be happy to
reconsider this.)

yamllint/conf/relaxed.yaml

@@ -25,3 +25,4 @@ rules:
     indent-sequences: consistent
   line-length:
     level: warning
+    allow-non-breakable-inline-mappings: yes