feat: Add generator_args arg to populate --generator-args artman option

[tswast]

This enables the possibility of sample generation via the --dev_samples
option to the gapic generator. Tested with the Data Catalog Python
client via

library = gapic.py_library(
    "bigquery_datatransfer",
    version,
    config_path="/google/cloud/bigquery/datatransfer/"
    "artman_bigquerydatatransfer.yaml",
    artman_output_name="bigquerydatatransfer-v1",
    include_protos=True,
    generator_args=["--dev_samples"],
)

Closes #223

[tswast]

Tests are passing locally.

==================================================================== 37 passed in 1.67 seconds =====================================================================
nox > Session test was successful.
nox > Running session generate_protos
nox > Creating virtualenv using python3.6 in /Users/swast/src/synthtool/.nox/generate_protos
nox > pip install --upgrade grpcio-tools
nox > python -m grpc_tools.protoc -Isynthtool/protos --python_out=synthtool/protos synthtool/protos/metadata.proto
nox > Session generate_protos was successful.
nox > Ran multiple sessions:
nox > * blacken: success
nox > * lint: success
nox > * test: success
nox > * generate_protos: success

Just pushed a commit for blacken.