Prefer to avoid common-lisp API, stick to simple emacs lisp.

This is slightly more efficient than calling symbol-value
on every dictionary.

Resolve #19

Multiple dictionary support

Closes #9

See merge request !6

This commit contains the necessary changes and implementation for
simultaneously using multiple independent dictionaries, and the
ability to extend spell-fu with new types of dictionaries.

Changes:

- Add support for multiple dictionaries.

  This allows e.g. enabling dictionaries for multiple languages
  simultaneously, within the same buffer.

- Define a simple API for dictionaries.

  This allows users or third-party packages to define dictionaries
  which are backed by sources other than aspell/ispell.

- Extract Ispell/Aspell dictionary support into its own section.

  Refactors Ispell/Aspell-related logic into an encapsulated
  dictionary implementation.

- Extract the personal dictionary into a separate logical dictionary.

  Aside the pedantic improvement of separation of concerns, this
  avoids the need to rebuild spell-fu's representation of the
  dictionary when the user adds/removes words from their personal
  dictionary.

  Combined with the changes above, this also allows having multiple
  personal dictionaries (e.g. per-language or per-project); when
  multiple personal dictionaries are enabled, spell-fu will ask the
  user which dictionary words should be added to.

  For simplicity, some code is currently repeated between the Ispell /
  Aspell and personal dictionaries.

- Regenerate spell-fu's cache when the aspell dictionary has changed.

  Caches were erroneously only cleared for personal dictionaries.

Place all Ispell / Aspell logic into a single section.

This commit's purpose is mainly to make the diff of the following
commit readable.

- Replace spell-fu--cache-table, a single hash table, with
  spell-fu--cache-tables, a list of hash tables

- Replace spell-fu--ensure-dict with spell-fu--ensure-dicts, and make
  it take a list of dictionary names

- Replace spell-fu--dictionary with spell-fu--dictionaries, which for
  now returns a single dictionary

- Update related code. For now, spell-fu--word-add-or-remove assumes
  the given word comes from the first dictionary.

When spell-fu minor mode wasn't enabled, these commands did nothing.

Now an error is raised instead.

This made single quoted strings detect "'word'" as "word'"
which typically caused the it to show as misspelled.

Now it's detected as "word".

Change due to personal preference.

Faces no longer split words when neither face is ignored.

No need to check the faces for each word as the face property
is already being stepped over.

Skip all buffers that use a mode derived from 'special-mode',
which is used for application buffers (not text).

(spell-fu-mode) can still be enabled explicitly, it just won't be turned
on for special modes by default when `global-spell-fu-mode` is used.

Resolves #2