This was previously not supported; todo_completion always used plain todo.sh from the PATH. Now it's possible to override the used executable (and configuration used through _todo_sh, set from a completion function wrapper.
Cp. http://tech.groups.yahoo.com/group/todotxt/message/4003
Replace the primitive substitutions inside the completion function with a call to sed (instead of calling head), and supply more powerful substitutions there.
This enhancement to todo_completion requires a small enhancement to the listfile action: When no SRC is specified, the list of text files in the todo.txt directory is printed. This is probably also useful on its own, and better than the original behavior of printing "TODO: File does not exist."
Note: I intentionally omitted bullet-proof error handling ($TODO_DIR non-existing or no text files contained), to avoid over-complicating this.
This is useful for the paranoid before a destructive todo.txt operation. Appending the text as a shell comment doesn't affect the todo.txt command itself, but shows that the task number corresponds to the task you had in mind.
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.
