feat: header is not mandatory to narrow pre block

docs: fix typos and document new features in README.md

fix: clickable URL take multiple lines while editing

refactor: reorder function for reading code to to bottom

refactor: do not discriminate heading levels

feat: can narrow pre blocks

User can `C-c C-c` in a pre blocks to open a new buffer with the
contained text and the adequate major mode. `C-c C-c` again to go back
to the Gemtext buffer.
Known issue: there must be at least one empty line in the pre block,
otherwise the command will not execute properly.

Kyle Anders <kyle@posteo.net> is the main author of this contribution.

feat: add support for yanking media feature

Co-authored-by: Kyle Andrews <kyle@posteo.net>
Co-authored-by: Antoine Aubé <courriel@arjca.fr>

fix: some URL are not clickable

Remove the use of goto-address-mode because it does not handle links like
"=> my_img.png". Implemented solution is inspired from markdown-mode.

doc: add MELPA installation in README

fix: @riscy review

* `(< (point) (point-max))` is equivalent to `(not (eobp))`
* remove redundant mode name setting
* remove occurences of `add-hook` on `gemtext-mode`

https://github.com/melpa/melpa/pull/8802#issuecomment-1784202217

fix: links regular expression does not match all URL

feat: initial commit

* Add a README.md with installation instructions and features
* Add a CONTRIBUTING.md
* Add the GNU GPLv3 license file
* Add the elisp file with basic features of the mode: highlighting and some editing utilities