[Fix #421] Use source for python-flake8
Update Texinfo manual
Refactor Emacs Lisp checking forms

Don't splice the source file into the check forms, and instead use
Flycheck's standard substitution mechanism to pass the sources as
arguments.

Reset command-line-args-left to avoid #319
Extract common code from emacs lisp and checkdoc checkers

Add flycheck-prepare-emacs-lisp-form which wraps up the common setup
from both checkers.
Simplify predicates of Emacs Lisp checkers
Fix test case

Regression from e9d711b
[Fix #423] Demote package-initialize errors

In emacs-lisp, demote errors from package-initialize to prevent the
syntax checking process from exiting early with failure in case
package-initialize errors, e.g. due to invalid autoloads or missing
requires.