add default_complete option

[casperdcl]

Fixes #65 /CC @tshu-w

[tshu-w]

@casperdcl Hi, I try this commit but it doesn't work on my side.
Steps:

1. install shtab

pip install git+https://github.com/iterative/shtab.git@default_complete

2. change code to use DEFAULT_FUNCTIONS:

shtab.add_argument_to(self.parser, ["-s", "--print-completion"], default_complete=DEFAULT_FUNCTIONS)

3. generate completion script:

run -s zsh

However, the script doesn't contains _default either.

……
_shtab_run_tune_options=(
  "(- :)"{-h,--help}"[show this help message and exit]"
  {-c,--config}"[Path to a configuration file in json or yaml format.]:config:"
  "--print_config[Print the configuration after applying all other arguments and exit.]:print_config:"
  "--seed_everything[Set to an int to run seed_everything with this value before classes instantiation]:seed_everything:"
  "--trainer[Path to a configuration file.]:trainer:"
……

[tshu-w]

Here is the minimal reproduction:

1. pip install git+https://github.com/iterative/shtab.git@default_complete

2. create test.py

#!/usr/bin/env python
import argparse
import shtab  # for completion magic

def get_main_parser():
    parser = argparse.ArgumentParser(prog="pathcomplete")
    shtab.add_argument_to(parser, ["-s", "--print-completion"], default_complete="_default")  # magic!
    # file & directory tab complete
    parser.add_argument("file", nargs="?")
    parser.add_argument("--dir", default=".")
    return parser

if __name__ == "__main__":
    parser = get_main_parser()
    args = parser.parse_args()
    print("received <file>=%r --dir=%r" % (args.file, args.dir))

3. print completion

λ python test.py -s zsh
#compdef pathcomplete

# AUTOMATCALLY GENERATED by `shtab`


_shtab_pathcomplete_commands() {
  local _commands=(

  )
  _describe 'pathcomplete commands' _commands
}

_shtab_pathcomplete_options=(
  "(- :)"{-h,--help}"[show this help message and exit]"
  {-s,--print-completion}"[print shell completion script]:print_completion:(bash zsh tcsh)"
  "--dir[]:dir:"
)


_shtab_pathcomplete() {
  local context state line curcontext="$curcontext"

  _arguments -C $_shtab_pathcomplete_options \
    ': :_shtab_pathcomplete_commands' \
    '*::: :->pathcomplete'

  case $state in
    pathcomplete)
      words=($line[1] "${words[@]}")
      (( CURRENT += 1 ))
      curcontext="${curcontext%:*:*}:_shtab_pathcomplete-$line[1]:"
      case $line[1] in

      esac
  esac
}



typeset -A opt_args
_shtab_pathcomplete "$@"