Resolves: #36

This PR adds two variables g:suda#try_no_text_input_auth and g:suda#try_no_text_input_auth_prompt that make it possible to try authentication methods that require no text input before prompting for a password. Other included changes are listed below:

• unsilent is prepended to :echo and :echomsg calls because of this reason. Without unsilent messages aren't shown when this plugin handles file reading.
• dd is used instead of cat with shell redirection to read a file in Unix-like operating systems and write it to the temporary files. This change is introduced because even if called with -p '', sudo may use PAM modules that write messages to stdout (like pam_fprintd.so in Linux), which, when cat with shell redirection is used, also get redirected into the temporary file. dd can write to the temporary file directly, so it can solve the aforementioned problem. I think it is safe to assume temporary files created by calling writefile() on tempname() is writable when using sudo.
• :echohl None is added to the end of s:echomsg_exception().
• s:strip_prefix() removes all suda:// prefixes instead of just one, so both :write and :SudaWrite work for buffers opened using :SudaRead.