Allow expiretiles_zoom to be set in config JSON

[flother]

Before this change, the default value for the -expiretiles-zoom command-line flag, used by the diff and run commands, would always overwrite whatever value was set for expiretiles_zoom in the config JSON. This change allows for four options:

• No flag value, no config value: default value used
• No flag value, config value: config value used
• Flag value, no config value: flag value used
• Both flag and config value: flag value used

Fixes omniscale/imposm3#181.