Unify scripts and add console script entry point

[aluttik]

With this change there will be one executable that gets copied into PATH on pip install called crossplane. It will make the various scripts into subcommands. This addresses Issue #4.

$ crossplane --help
usage: crossplane <command> [options]

various operations for nginx config files

optional arguments:
  -h, --help            show this help message and exit

commands:
  parse                 parses a json payload for an nginx config
  lex                   lexes tokens from an nginx config file
  minify                removes all whitespace from an nginx config
  format                formats an nginx config file
  help                  show help for commands

$ crossplane parse --help
usage: crossplane parse [-h] [-o OUT] [-i NUM] [--no-catch] [--tb-onerror]
                        filename

parses a json payload for an nginx config

positional arguments:
  filename              the nginx config file

optional arguments:
  -h, --help            show this help message and exit
  -o OUT, --out OUT     write output to a file
  -i NUM, --indent NUM  number of spaces to indent output
  --no-catch            only collect first error in file
  --tb-onerror          include tracebacks in config errors

$ crossplane lex --help
usage: crossplane lex [-h] [-o OUT] [-i NUM] [-n] filename

lexes tokens from an nginx config file

positional arguments:
  filename              the nginx config file

optional arguments:
  -h, --help            show this help message and exit
  -o OUT, --out OUT     write output to a file
  -i NUM, --indent NUM  number of spaces to indent output
  -n, --line-numbers    include line numbers in json payload

$ crossplane minify --help
usage: crossplane minify [-h] [-o OUT] filename

removes all whitespace from an nginx config

positional arguments:
  filename           the nginx config file

optional arguments:
  -h, --help         show this help message and exit
  -o OUT, --out OUT  write output to a file

$ crossplane format --help
usage: crossplane format [-h] [-o OUT] [-i NUM | -t] filename

formats an nginx config file

positional arguments:
  filename              the nginx config file

optional arguments:
  -h, --help            show this help message and exit
  -o OUT, --out OUT     write output to a file
  -i NUM, --indent NUM  number of spaces to indent output
  -t, --tabs            indent with tabs instead of spaces