Use "local" to avoid that the internal completion variables are accessible from the user's shell.
Use "todo.sh command" for the context/project lookups to avoid interference with custom add-ons of the same name, and reset TODOTXT_VERBOSE to avoid adding any message output (currently there is none).
The shebang is ignored when sourcing the script (but still helps many text editors auto-detect the file type), and will cause an error when the script is mistakenly executed.
The Bash check allows to have this called from a generic place (e.g. .profile), and do no harm when under a different shell.
Don't infringe against the principle of least astonishment (they user may have completely unrelated aliases). Rather, if the user sets up his own alias, make him apply the same to todo_completion.
