Build profile/configuration

[easeway]

Allow HyperMake to accept configuration which is a collection variables and can be referenced in targets as environment variables. A configuration with preset variables is called build profile. Here's the example in settings section:

settings:
    variables:
        var1:
            description: what's var1
            default: default value
            required: true/false
            env: environment variable name in targets, default is HMAKE_VAR_VAR1
        var2:
        ...
    profiles:
        profile1:
            var1: value1
            var2: value2
        profile2:
            var1: value1.1
            var2: value2.2
    default-profile: profile1

Introduce --profile=profile-name, -o profile-name to select pre-defined profile.
Introduce --define var=val, -D var=val to define a variable

[easeway]

Move out of milestone as not yet determined if this feature is valuable without introducing too much complexity.