Add version callback and more help text

oxo/__init__.py

@@ -1 +1 @@
-__version__ = "0.1.0"
+from oxo._version import __version__

oxo/__main__.py

@@ -4,11 +4,31 @@ from pathlib import Path
 import httpx
 import typer
 
+from oxo import __version__
+
 app = typer.Typer()
 
 base_url = typer.Option("http://0x0.st", envvar="OXO_BASE_URL")
 
 
+def version_callback(value: bool):
+    if value:
+        typer.echo(f"{__version__}")
+        raise typer.Exit()
+
+
+@app.callback()
+def main(
+    version: t.Optional[bool] = typer.Option(
+        None, "--version", callback=version_callback, is_eager=True
+    ),
+):
+    """A command line utility for 0x0.st compliant pastebins.
+
+    Set `OXO_BASE_URL` if the base URL is not http://0x0.st.
+    """
+
+
 @app.command()
 def files(
     files: t.List[Path] = typer.Argument(