Add missing query parameters to GetDiff

[CescHolly]

Changes:

Added the additional fields:

• context
• path
• from_pullrequest_id
• ignore_whitespace
• binary
• renames
• topic

to GetDiff.

Justified by the documentation page, and adding from_pullrequest_id as it appears in pullRequest.Links.Diff.Href:

"diff": {
    "href": "https://api.bitbucket.org/2.0/repositories/OWNER/REPO_SLUG/diff/SPEC?from_pullrequest_id=3&topic=true"
}

When using GetDiff with from_pullrequest_id (as also happens with GetDiffStat) the SPEC would follow the format: REPO_SLUG:hash.

NOTE: I am aware that the DiffOptions type is also used as a parameter to GetPatch, which does not seem to support all these query parameters. I was tempted to create a new type:

type PatchOptions struct {
	Owner             string `json:"owner"`
	RepoSlug          string `json:"repo_slug"`
	Spec              string `json:"spec"`
}

but replacing the current parameter in GetPatch with this new type would break backwards compatibility with any current consumers of this library.

So I just mention it in case you want to do that separately in a future version change ;)

[ktrysmt]

LGTM!