Remap j and k to move into wrapped lines

Normally j and k move over wrapped lines and onto the next actual line
in the file. If a line is wrapped because the window is too narrow, the
wrapped portion of the lines will be skipped and you need to press gj/gk
to move among the wrapped portions. Normally we want to move into
wrapped lines, so remap j and k to do this.
Allow edited buffers to be hidden without saving

Without this option, vim complains that you have unsaved changes in a
buffer before it will let you swap to a different buffer.
Set the filetype for *.md files to markdown

Vim's default runtime files were setting the filetype for files with the
*.md file extension to modula2. We don't write modula 2, but we do write
markdown and use the *.md extension. Adding this ftdetect file will
override the system settings and make us happy.