Add configurable options support for redpen linter (#5006)

Redpen linter previously had hardcoded command-line options and duplicated
linter definitions across all supported file types (asciidoc, markdown,
review, rst, tex, text). This refactoring centralizes the linter definition
and adds support for user-configurable options via g:ale_redpen_options.

Key changes:
- Created ale#handlers#redpen#DefineLinter() to eliminate code duplication
- Added ale#handlers#redpen#GetCommand() to support configurable options
- All file types now use shared configuration and command building
- Added comprehensive test coverage for option handling
- Updated documentation for all affected file types

This allows users to customize redpen behavior with additional command-line
options while maintaining backward compatibility and reducing maintenance
overhead.

test/linter/test_redpen.vader

@@ -0,0 +1,29 @@
+Before:
+  " This is just one example of a language using the linter.
+  call ale#assert#SetUpLinterTest('markdown', 'redpen')
+
+  unlet! g:ale_redpen_options
+  call ale#Set('redpen_options', '')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The options should be omitted by default):
+  AssertLinter
+  \ 'redpen',
+  \ 'redpen -f markdown -r json %t'
+
+Execute(The options should be used in the command):
+  let g:ale_redpen_options = '--foo --bar'
+
+  AssertLinter
+  \ 'redpen',
+  \ 'redpen -f markdown -r json --foo --bar %t'
+
+Execute(The command should work with different filetypes):
+  " Test with a different filetype
+  call ale#assert#SetUpLinterTest('text', 'redpen')
+
+  AssertLinter
+  \ 'redpen',
+  \ 'redpen -f text -r json %t'