Add configurable page title via dedicated Title option

[nikicat]

Make the Swagger UI HTML page title configurable.

Adds a Title config field and a Title(...) functional option. It defaults to "Swagger UI", so the page title is unchanged unless explicitly set:

e.GET("/swagger/*", echoSwagger.EchoWrapHandler(echoSwagger.Title("My API")))

This replaces the earlier {{.InstanceName}} | Swagger UI approach. A dedicated field is cleaner (per @svachaj's review feedback) and avoids regressing the default title to swagger | Swagger UI for users who don't set an instance name.

Related to #115

[svachaj]

InstanceName doesn't always have to be the best option for the page title. How about creating a new property, PageTitle? I think it's cleaner.

[nikicat]

@svachaj good point — reworked accordingly. Instead of reusing InstanceName, this now adds a dedicated Title config field with a Title(...) option, defaulting to "Swagger UI" so the default page title is preserved. Thanks for the suggestion!