Make 'image' crate optional

[xamgore]

Discussion at #763.

#[derive(Debug, Clone, Serialize)]
#[serde(rename_all = "camelCase")]
pub struct Docx {
   ...
    // reader only
    pub images: Vec<(String, String, Image, Png)>,
}

Is Png really needed here? Why not just Image? The end user may convert buffer themselves…

[bokuweb]

Is Png really needed here? Why not just Image? The end user may convert buffer themselves…

I wanted to convert TIFF to PNG using WebAssembly for use with docx-wasm in the browser (This is because browsers don't support tiff), but at least for native usage, the Image crate works well. It seems that I can reconsider the approach for WebAssembly as well.

[bokuweb]

LGTM