Add support for Markdown.

I often use `ag` to sift through google/WebFundamentals (a huge pile of Markdown files), but having to do `ag -G '.+\.markdown'` makes me sad. I'd much rather use `--markdown`, which is why I'm submitting this patch.

Tests appear to pass:

    ~/Code/the_silver_searcher[add-markdown-support]% make test
    cram -v tests/*.t
    tests/bad_path.t: passed
    tests/case_sensitivity.t: passed
    tests/exitcodes.t: passed
    tests/hidden_option.t: passed
    tests/ignore_abs_path.t: passed
    tests/ignore_absolute_search_path_with_glob.t: passed
    tests/ignore_backups.t: passed
    tests/ignore_examine_parent_ignorefiles.t: passed
    tests/ignore_pattern_in_subdirectory.t: passed
    tests/ignore_subdir.t: passed
    tests/invert_match.t: passed
    tests/list_file_types.t: passed
    tests/passthrough.t: passed
    tests/vimgrep.t: passed
    # Ran 14 tests, 0 skipped, 0 failed.