Changed Frontmatter regex pattern to allow for CRLF line endings.

When creating pages or posts on the windows platform the .md files will use the CRLF line ending by default. Currently the regex in src/Yosymfony/Spress/ContentManager/Frontmatter.php only checks for the LF line ending which means that posts and pages do not appear to have frontmatter and will not be processed.

Altered the regex to account for both line ending types and now both CRLF and LF formatted files are processed.