Allow display of images alternate text

# Issue details

### Duplicate?
Have you searched the issues of this repository if your issue is already known? yes; no result with "alt" or "alternate"

### Actual behaviour

On articles from xkcd.com, there are alternate text on pictures, that are really part of the fun; they're used as some kind of hidden spoiler for irony or so. Other example (french): https://unodieuxconnard.com/

If an article is saved in my wallabag, there is no way to display the alternate text.

### Expected behaviour

I would like to be able to display the alternate text, e.g. by long click on a picture, with some sort of toast display; then at a new click, to get the toast removed.

# Steps to reproduce the issue
1. Add [this article](https://unodieuxconnard.com/2018/08/27/je-parle-le-wesh/) to your bag
2. Synchronize the android wallabag app
3. Open the article, scroll to show the (second) picture.
4. Tapping the picture proposes options for the image, but not the alternate texte (« Avec des yamakazis, bien sûr »). A long tap does nothing.

# Environment details
- **wallabag app version**: 2.1.0-beta
- **wallabag app installation source** (e.g. Gplay, F-Droid, manual): Google Play
- **Android OS version**: 6.0.1
- **Android ROM** (e.g. stock, LineageOS, SlimRom,…): Stock Sony/Docomo android
- **Android hardware**: Sony Z3 Compact Docomo (SO-02G)
- **wallabag server version**: unknown : it is current framabag.org installation
- **Do you have Two-Factor-Authentication enabled?**: unknown

# Logs

The server is framabag.org, so no idea how to access any log/data.

# Your experience with wallabag Android app
Have you had any luck using wallabag Android app before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

--> Yes it works, it's nice-looking, integration is flawlessly and doc is complete. Thanks for the very good job !

No source files or tests are named. Start by locating the Android article-reading image interaction and how image alternate text is loaded, then reproduce the linked article's second image. Done means a reader can reveal and dismiss the image's alternate text through a documented interaction.

Written by the indexing model from the issue text.