Add PkgImporter to vm.Options for handling import statement

[ipsusila]

If specified, it will handle import statement through vm.Importer interface. This patch adds capability to specify only certains std packages are available to the script. For backward compatibility, if Options or Options.PkgImporter is not specified, vm will looking for package/types from env.Packages and env.PackageTypes.

Use cases example:

// 1. "github.com/mattn/anko/packages" is not imported
// No packages will be available for import
_, err := vm.ExecuteContext(ctx, en, nil, src)
_, err := vm.ExecuteContext(ctx, en, &vm.Options{Debug: true}, src)

// 2. "github.com/mattn/anko/packages" is imported explicitly
// All packages will be available for import statement
_, err := vm.ExecuteContext(ctx, en, nil, src)
_, err := vm.ExecuteContext(ctx, en, &vm.Options{Debug: true}, src)

// 3. "github.com/mattn/anko/packages" is imported explicitly
// All packages will be available for import statement. 
// Access are safe for concurrent usage.
_, err := vm.ExecuteContext(ctx, en, &vm.Options{PkgImporter: vm.NewStdPackagesImporter()}, src)

// 4. "github.com/mattn/anko/packages" is imported explicitly
// Only package `fmt` and `time` will be available for import
_, err := vm.ExecuteContext(ctx, en, &vm.Options{PkgImporter: vm.NewStdPackagesImporter("fmt", "time")}, src)

// 5. Import custom package, no std packages
_, err := vm.ExecuteContext(ctx, en, &vm.Options{PkgImporter: vm.NewPackagesImporter(pkg, pkgTypes)}, src)

// 6. "github.com/mattn/anko/packages" is imported explicitly
// Import custom package AND std packages
_, err := vm.ExecuteContext(ctx, en, &vm.Options{PkgImporter: vm.NewPackagesWithStdImporter(pkg, pkgTypes, "fmt", "io", "time")}, src)

Probably related to issue #358