Inspired by https://github.com/todotxt/todo.txt

-----< Basic concept >-----
The program loads and operates on one todo file at a time.
A todo file is a text file where (with a few exceptions) every line is one todo item.

-----< Functionality >-----
- load file (parsing)
- save file

- prune completed tasks

- "running tasks":
Two-line structure that contains:
1. A line to determine when, and set variables (both using calendar formulas)
2. A "task recipe" that can use $variables (for birthdays etc.)
Running tasks are computed in the order they appear in the file.

-----< How it works >-----
1. $ todo work.todo
2. load the entire file
	2.1 parse lines
	2.2 preserve structuring of the file: empty lines, comments, order
	2.3 if there's any errors, abort instead of possibly overwriting the file with wrong data later
3. calculate any running tasks
	3.1 look at the last-calculated date, current date and task duration
	3.2 calculate new tasks and append them to the file
	3.3 update last-calculated date of the task
	3.4 save updates
4. present user interface (TUI)
	4.1 show tasks
	4.2 search and order tasks
	4.3 check tasks
	4.4 Optional: delete tasks
	4.5 Optional: create new tasks
	4.5 Optional: edit existing tasks (modify text, priority, ...)
5. overwrite file with new changes
	5.1 instant mode
		5.1.1 after initial running task calculation
		5.1.2 whenever any task was changed (checked, edited, deleted, ...)
	5.2 deferred mode
		5.2.1 save on user save action
		5.2.2 save on close
	5.3 default mode: instant mode?

-----< The file format >-----
normal tasks:
[x] [priority] [[completion-date] creation-date] text
where completion-date can only be specified if the task was actually completed

running tasks:
# [running-date] when-formula template

text:
- may include any amount of +projects and @contexts
	(+projects and @contexts are case insensitive)
- may include key:value specifiers
	(key and value must consist of non-whitespace characters which are not colons)
	(keys are case insensitive too)

template:
- consists of any string
- may include {expressions} which are when-formulas
	(if the when-formula returns a date, the expression is replaced by its YYYY-MM-DD representation)
	(if the when-formula returns a number, the expression is replaced by that number)
	(if the when-formula returns a boolean, the expression is replaced by "yes" or "no")

-----< Examples/Testing >-----
The program should work with all https://github.com/todotxt/todo.txt compliant todo files.
"Running tasks" are merely an extension of the todo format.
They use a syntax similar to what I expect comments would look in todo.txt 2.0.
In todo.txt compliant programs, they simply show up as not completed tasks.

Starting the program
 - read command line options
 - export default config
 - export default theme
 - load config file, looking in default places if no -c flag set
 - merge loaded config with options (options take priority)
 - open db and make sure the correct tables exist (CREATE TABLE IF EXISTS)
  (- possibly import from old table versions)
 - launch actual program

Actual program
 - print all entries in db for debugging