loading fasd clobbers z(), and results in a cryptic syntax error
message, since 'z' is already an alias when z.plugin.bash is being
sourced.
this conditional detects this situation and returns early.
add about-plugin metadata
chmod -x plugins
cleanup filenames to standardize on x.plugin.bash format
only plugin files intended to be executable from the command line should
contain a shebang line, and should be a+x.