HSTS⇒maxAge in config must not be a string

[Nebukadneza]

Hi,

with the recent docker image update, the docker fails to start with the sample config, and i guess many user configs too, with:

app_1_c5e3955ff34f | 2019-03-04T12:22:45.572362083Z 2019-03-04T12:22:45.571Z error: 	uncaughtException: HSTS must be passed a numeric maxAge parameter.
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572386521Z TypeError: HSTS must be passed a numeric maxAge parameter.
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572391266Z     at Function.hsts (/codimd/node_modules/hsts/index.js:17:11)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572394590Z     at Object.<anonymous> (/codimd/app.js:85:18)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572397977Z     at Module._compile (module.js:653:30)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572400989Z     at Object.Module._extensions..js (module.js:664:10)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572403913Z     at Module.load (module.js:566:32)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572406751Z     at tryModuleLoad (module.js:506:12)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572409738Z     at Function.Module._load (module.js:498:3)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572412867Z     at Function.Module.runMain (module.js:694:10)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572415813Z     at startup (bootstrap_node.js:204:16)
app_1_c5e3955ff34f | 2019-03-04T12:22:45.572418859Z     at bootstrap_node.js:625:3

Fix is easy: remove the " around the maxAge parameter in the config. It would be great if at least the sample config adapted to the hsts module handling of the config-value …

As always, thanks for the great project ^_^