Optionally use stdin and stdout for validate, describe, and convert commands

[tschaub]

This makes it so the commands optionally read from stdin and write to stdout.

Examples:

# validate geoparquet from stdin
cat example.parquet | gpq validate

# describe geoparquet from stdin
cat example.parquet | gpq describe

# convert geoparquet from stdin to geojson stdout
cat example.parquet | gpq convert --from geoparquet --to geojson

# convert geojson to geoparquet stdout
gpq convert example.geojson --to geoparquet

Fixes #78.

[bdon]

Tested locally, LGTM