Improve java checkstyle command configuration.

[hsanson]

The g:ale_java_checkstyle_options only worked with absolute paths making
it difficult to have a per project configuration. This MR introduces a
new g:ale_java_checkstyle_config configuration. This option can be an
absolute path or a filename. When a filename is set then the nearest
file that matches it in the current buffer folder or any of its parents
will be used.

Note: The above is a breaking change since it replaces the
g:ale_java_checkstyle_options variable with a different one.

Also a new configuration g:ale_java_checkstyle_executable is added. This
allows users to create more specialized launch scripts (#1305) to invoke
checkstyle.

[w0rp]

The new options are fine, but keep the existing _options setting and change the default to ''. Some people will be using that option already. Check if the options contain -c, and use that instead if the configuration file is specified through the options, for maintaining backwards compatibility. We could add a deprecation warning so we can remove it later.

[hsanson]

@w0rp Added the _options setting back. If defined I use this setting to build the command and ignore the new ones. If this option is defined is expected to have '-c' so no need to check for it. Also mentioned in the docs that the other settings are preferred over this one.

[w0rp]

I rewrote this in the latest commit. It should support absolute paths for the configuration file in the new _config option, relative paths, prefer -c in _options if the configuration file is set in the options, and the executable is now configurable.