Allow port to be configured

Adds ability to configure port via AppOpts or setting PORT=N environment variable.

Example:

h.Start(h.AppOpts{
		ServiceLocator: locator,
		Port:           3005,
		LiveReload:     true,
})

or

PORT=3005 htmgo watch

If you set both AppOpts.Port and PORT env variable, AppOpts.Port will take priority.

to update:

go get github.com/maddalax/htmgo/[email protected]