Take command-line argument as ivnentory file name

Introduce line: namespace

- Add words line:empty? and line:dash? to be able to recognize lines that
  carry no information.

- Rename load-line to line:load to keep line-related word names consistent.

Implement basic time operations

Since RETRO doesn't support a great deal of time operations, implement a
set of words to parse dates into numbers of days since 1999.12.31.

Move s:trim to load-line

- Make code less redundant by trimming the input earlier.
- Make the test output a bit more comprehensible.

Set up a repository, add LICENSE and README