Allow RegEx and Strings for Showmore plugin

[hagenburger]

Currently the Showmore plugin accepts ranges (= from line X to line Y).

In future, it should also support RegEx and Strings like the Highlight plugin. For example:

```html +shomore=/value=".+"/
<input class="my-class" value="Lorem ipsum">

Should look like:

```html
<input class="my-class" ···>

On click, the ··· button should reveal value="Lorem ipsum".

Further reading:

• https://github.com/hagenburger/pimd/tree/master/plugins/showmore#pimd-showmore-plugin: Current implementation of the Showmore plugin with video example of revealing the hidden content
• https://github.com/hagenburger/pimd/tree/master/plugins/highlight#pimd-highlight-plugin: how the strings and RegExp are implemented in the Highlight plugin (instead of highlighting the match, this issue should replace the match with a ··· button)
• https://github.com/hagenburger/pimd/blob/master/plugins/showmore/index.js: This is the place where the code should be implemented
• https://github.com/hagenburger/pimd/blob/master/plugins/showmore/test.js: This is the place where the tests should be implemented