Markup Songbook Format specification

This file defines a "standardized" plain-text format intended as an
intermediate format for creation of songbooks. This format should be:
- human readable
- easy to edit via a text editor
- easy to process by text-oriented and file-system oriented tools
- should allow easy sharing of song databases, as well as easy merging and
  mixing of different song databases

We define two versions of the format: strict and nonstrict. The strict version
adds some further restrictions which should simplify creation of export tools.

Quick, self-defining example:
  ##title: Drunken Sailor
  ##interpret: Irish Rovers
  ##author: unknown
  ==============
  [Ami]What shall we do with a drunken sailor
  wh[G]at shall we do with a drunken sailor
  wh[Ami]at shall we do with a drunken sailor
  early [Emi]in the m[Ami]orning?

  R: H[Ami]ooray and up she rises h[G]ooray and up she rises
  h[Ami]ooray and up she rises early [Emi]in the m[Ami]orning.

  Give 'im a dose of salt and water...

  Stick on his back a mustard plaster...

  Keep him there and make 'im bail 'er...

  #Commented out, this song would otherwise bee too long...
  #Put/chuck him in the long boat 'til he's sober...
  #
  #Put him in the long-boat and make him bail her...
  #
  What shall we do with a drunken sailor?..


Non-strict definition:
- Each file contains exactly one song. Name of this file is intended to
  determine alphabetic ordering of songs. File names are also should be
  file-system friendly. E.g., they should use only standard ascii letters,
  digits and underscores. 
- The content should contain the following sections:
  - Metadata section: Set of records specified one per one line in format
    '##<key>: <value>'. Keys may be:
    - title (compulsory, should be on the first line)
    - interpret (optional)
    - author (optional)
    - any other fields the user finds desirable, including possible variable
      values for exporting tools and such information. General keywords may be
      used in future. 
  - Underline: three or more equality signs denoting that all metadata has been
    specified.
  - Text of song:
    - Chords are specified in standard bracket notation.
    - Lines started by '#' are considered comments. If left out, the file
      should still comply with the format specification.
    - Verses and chorus are not distinguished. Every paragraph (be it verse,
      chorus or a text note) is considered to be one verse.
    - Empty lines determine end of the previous verse and beginning of the next
      verse.

Strict definition:
- All of the non-strict requirements apply.
- Unneccessary white spaces are forbidden. I.e., line should not end by a space.
- Two or more consecutive empty lines are forbidden.
- Empty line at the end of file is forbidden.
- Empty line between unerline and the first verse is forbidden.
- All characters should be tex-parsable as long as utf8 input encoding is
  specified.