External lint program

[ksoichiro]

golang is useful to create portable program, but it is not always appropriate to add some user-defined lint algorithms.
So fint should allow execution of external programs as modules.
Maybe it is necessary to add some variables like project root path, module root path, etc.

Directory structure:

.fint
  builtin
    modules
      something <-- external program's configuration
        config.json
        sth <-- external binary

something/config.json:

{"type":"external", "executable":["${module_root}/something/sth", "-s ${src_root}"], "description":"my original tool."}