[improvement] Use new picocli lib for CLI

[ferozco]

Similar to palantir/conjure-java#91. Replace usage of commons cli with picocli

ex:

Usage: conjure-python generate [-hV] [--rawSource] [--writeCondaRecipe] [--packageDescription=<packageDescription>]
                               [--packageName=<packageName>] [--packageUrl=<packageUrl>]
                               [--packageVersion=<packageVersion>] <input> <output>
Generate Python bindings for a Conjure API
      <input>              Path to the input IR file
      <output>             Output directory for generated source
      --packageDescription=<packageDescription>
                           The description of the package to generate.
      --packageName=<packageName>
                           The name of the package to generate.
      --packageUrl=<packageUrl>
                           The url of the package to generate
      --packageVersion=<packageVersion>
                           The version of the package to generate.
      --rawSource          Only generate the plain source without any package metadata
      --writeCondaRecipe   Only generate the plain source without any package metadata
  -h, --help               Show this help message and exit.
  -V, --version            Print version information and exit.

[iamdanfox]

picocli is so great!