*: package context support in Repository, Remote and Submodule

[mcuadros]

The following method has being added in order to support contexts in the main package, following the standard library pattern, instead of modifying the current methods, the following *Context were added:

type Remote
    func (r *Remote) FetchContext(ctx context.Context, o *FetchOptions) error
    func (r *Remote) PushContext(ctx context.Context, o *PushOptions) error

type Repository
    func CloneContext(ctx context.Context, s storage.Storer, worktree billy.Filesystem, o *CloneOptions) 
    func PlainCloneContext(ctx context.Context, path string, isBare bool, o *CloneOptions) (*Repository, error)
    func (r *Repository) PushContext(ctx context.Context, o *PushOptions) error
    func (r *Repository) FetchContext(ctx context.Context, o *FetchOptions) error

type Submodule
    func (s *Submodule) UpdateContext(ctx context.Context, o *SubmoduleUpdateOptions) error

type Worktree
    func (w *Worktree) PullContext(ctx context.Context, o *PullOptions) error

@JoshuaSjoding PTAL

[JoshuaSjoding]

Splendid! I think these API changes are sufficient for #483.