Add feature: Choose preferred scheme name.

[oscarssanchez]

Hi @helen,

Could you use this PR that aims to add the feature requested in https://github.com/helen/admin-color-schemer/issues/16 ?

I'm not sure if my approach is intuitive enough to the end user, so it would be great to read some feedback.

So this is what the functionality looks like now:

When no name is provided the scheme name defaults to Custom

Example of adding a name to the custom scheme.

Maybe outputting an error if no scheme name is provided would be a better approach, which would be also related to issue https://github.com/helen/admin-color-schemer/issues/7

Thank you!