Refactor: Rename functions.

Add data format and source to functions reading bad and good spellings.
In preparation of LanguageTool XML data support.
Add function for loading bad and good spellings from default sources.

Move some file logic from LaTeX to Lua side.
More verbose error messages in case of errorneous file operations.
Read plain list file into string at once to speed-up file operations.
Change default file names used by the spelling package.

Bad and good spellings are read from:

  bad      <jobname>.spb   -->   <jobname>.spell.bad
  good     <jobname>.spg   -->   <jobname>.spell.good

Text output is written to file:

           <jobname>.txt   -->   <jobname>.spell.txt
Implement reading bad strings from LanguageTool XML error reports.
Catch LanguageTool XML parse errors.

Prepend a general package error message pointing to the XML source (file).
doc: Describe LanguageTool support.
doc: Add explicit footnote containing misspellings on title page.
doc: Revise introduction.
doc: Revise sec:work-flow.
doc: Revise sec:wordlists.
doc: Revise sec:textoutput.
doc: Revise sec:bugs
Update CHANGES.
Update version number and copyright notice.
Add LuaDoc meta data to main Lua file for sake of completeness.