Allow inline blocks in Showmore plugin

[hagenburger]

Right now, the Showmore plugin allows to hide 1 or more lines of code.

New feature

Allow ranges to accept columns:

1. +showmore=2:10..2:20: Hide character 10 to 20 in line 2
2. +showmore=2:10..20: Hide character 10 to 20 in line 2
3. +showmore=2:10..3:20: Hide character 10 in line 2 to 20 in line 3

Example use case

Additional attributes which are not important for the code:

```html +showmore=1:24..35
<input class="my-field" type="text">
```

Expected result

<input class="my-field" ···>