Add option to set the default provider in gupm.json

[azukaar]

Currently, when no provider is added, gupm stays the default provider. An option in gupm.json should allow user to set another provider as the default.

# gupm.json
{
  "cli" : {
     "defaultProvider": "npm"
   }
}

The option to set it only for certain tasks should also be available:

# gupm.json
{
  "cli" : {
     "make" : {
       "defaultProvider": "npm"
     }
   }
}

doing so, using g make will actually be equivalant to g make -p npm

Finally, a default provider for dependency retrieval:

# gupm.json
{
  "dependencies" : {
     "defaultProvider": "npm",
     "default": {
         .......
     } 
   }
}

[azukaar]

Code done -- need doc then issue can be closed