Resolve "List items"

Closes #7 (closed)

It is now possible to get formatted lists of items: cornerstone list --format=FORMAT.

Available placeholders

1. %i - ID of the to-do item
2. %t - title of the item

Placeholders escaping

\%PLACEHOLDER, where PLACEHOLDER is i or t

Available special sequences

1. \t pastes a horizontal tabulation
2. \n inserts an end-of-line character

Sequences escaping

\\CHARACTER, where CHARACTER is t or n