Pass the value of variables to checkers

Analogous to `config`, add `option` tag to checker commands to pass the value of a variable to a checker.  Accompany this tag with a macro `flycheck-def-option-var` that declares the corresponding variable.

# Interface

```scheme
:command '("phpcs" (option "--standard" flycheck-phpcs-standard) source)
```
The first element of `config` is the option to use, the second the variable symbol.

```scheme
(flycheck-def-option-var flycheck-phpcs-standard php-phpcs nil
   "The coding standard to use when checking with phpcs"
   :type 'string)
```

The first argument is the variable name, the second the associated checker, the third the default value, and the forth is the docstring.  Any further options will be pass to `defcustom`.  The variable will be made buffer local and customizable within a group `flycheck-checker-options`.

# Rationale

Allow to configure checkers beyond configuration files.  See #71 for an example.