Accept filters for listcon, too.

This has been implemented for listproj already, for consistency in use and implementation, the same should be available for listcon, too.
FIX: Mention filter capability in long help for listproj.

This was forgotten in the original enhancement in b824479.
test-lib: Avoid eating leading whitespace in expected output.

Currently, todo.sh produces no indented output, so this hasn't affected tests so far. I found this while testing one of my custom actions with the test-lib.
By unsetting IFS, the read command won't strip off leading and trailing whitespace while parsing the test session definition.
test-lib: Allow expected output with empty lines.

An empty line is used to delimit test commands in test_todo_session. (This wasn't properly reflected in the usage comment, fixed that, too.) This prevents us from testing commands that include empty lines. (Currently, there is no such output in todo.sh, but custom add-ons may have this.)
Instead of forcing the test to filter out the empty lines, extend the syntax to allow empty lines in the expected output by escaping them with a single backslash. (When redirecting via <<EOF, the backslash itself must be escaped = doubled; better use <<'EOF'.)
Refactoring: Extract listWordsWithSigil() from listcon and listproj.
Tests: Extract backtick_check().
Tests: Hard-code location of todo.sh.

As todo.sh is placed by test-lib at a fixed place, avoid the lookup through which.
Replace backticks in config with $(...)

This applies e24777fa2cf40d2a50b27478c841f1381f8f4a22 to todo.cfg, too, and adds a verification to the existing test.

Note: Because todo.cfg is preprocessed by test-lib, we use the original config from the working copy, not the copy inside the test fixture.
FIX: Replacement with priority and date doesn't replace existing date.
Refactoring: Extract duplicated regexp for priority and date.
Minor: Move cleaninput call closer to sed command.
FIX: todo_completion problem with Bash 3.1

Regexp matching changed from 3.1 to 3.2. Make it work for Bash 3.1 by using an intermediate variable, as in 394c4c748ac7fe9c4840536eb1ac28aa9a03853f.
Cp. http://tech.groups.yahoo.com/group/todotxt/message/4211
Allow use of post_filter_command for listall and listpri, too.

For ls / _list(), custom add-ons can use post_filter_command for additional filtering before the highlighting is applied. Let's allow that for listall and listpri, too, although they employ post_filter_command themselves.
Tests: Add test for listaddons.
Tests: Add test for ordinary help.
ENH: Print usage help for custom action.

Currently, the only way to get usage help for a custom action is to use "todo.sh help" and scroll / search for the action name. (Or try to call the action without / with invalid parameters to hopefully get a one-line syntax summary.)
This extends the help command to print the usage of optionally passed custom action names.
Refactoring: Extract make_action into actions-test-lib.sh.

This reduces the duplication in the individual test files.
ENH: Print usage help for all passed actions.

Extend the support for specific usage help to built-in actions.
FIX: Always return success after help.

Usually (i.e. when a pager is available), the help() output is piped through the pager, and then the return code will be the one of the pager. So the exit 1 at the end of help() doesn't make much sense, and should be abolished for consistency.
Add action argument for help to todo_completion.

Note that this is not perfect, as it only handles completing a single action after "help", but that should over the most important use case.