Allow sourcing dirconfig settings from custom configuration files

[attilayener]

Fixes #2197

Problem

Projectile can only parse project settings from its own .projectile
file. Projects that already carry a machine-readable configuration file
(e.g. an Eclipse .project' XML) cannot feed it into Projectile - the user has to maintain the same information twice, and pointing projectile-dirconfig-file' at such a file just makes the built-in
parser choke on its contents.

Solution

As proposed in #2197, this adds projectile-dirconfig-parsers: an
alist of configuration file names and parser functions. When a project
root has no projectile-dirconfig-file', the parser of the first file from the list found in the root is called with the file's absolute path and must return a projectile-dirconfig' structure (or nil).

• the built-in `.projectile' handling keeps precedence and is unchanged
• custom parsers can delegate to the new `projectile-read-dirconfig-file'
• parse results flow through the existing per-root cache; the cached
  entry records the resolved file's path and mtime, so edits invalidate
  it exactly like the built-in file
• the prefixless/glob `+' warnings now name the file the config was
  actually parsed from
• documented in doc/modules/ROOT/pages/ignoring.adoc
• covered by tests in test/projectile-ignore-test.el
• changelog entry added

;; example: parse an Eclipse .project file
(setq projectile-dirconfig-parsers
      '((".project" . my-eclipse-project-parser)))