Added a custom field to the ethconfig.Config to test deserialisation

[jp-imx]

To test, run:

make all
 ./build/bin/geth --config custom-config.toml 

Expected behaviour: toml file is deserialised successfully.

Actual behaviour:

./build/bin/geth --config custom-config.toml
INFO [09-05|12:43:39.710] Starting Geth on Ethereum mainnet...
INFO [09-05|12:43:39.711] Bumping default cache on mainnet         provided=1024 updated=4096
Fatal: custom-config.toml, line 23: field 'CustomField' is not defined in ethconfig.Config, see https://godoc.org/github.com/ethereum/go-ethereum/eth/ethconfig#Config for available fields