Print specific part of the Markdown by syntax

[marcelarie]

I've been using glow for a while now, and thought of a few cool features that could make it even better. Not sure if these fit the project's direction, but here they are:

1. Selective Header Printing: It'd be awesome to print specific headers directly. Something like:

glow --elements=header --patter-=User*

to get all element headers starting with "User".

The elements could be:

• heading
• code_block
• list
• link
• image
• blockquote
• table
• hr
• code
• emphasis
• task_list
• footnote
• math
• definition_list
• html

Another useful example would be to print only code blocks:

glow --elements=code_block

These ideas might be a bit scattered, but in a way I think are related.

2. Viewing Specific Document Sections: Like tail or head. Quick examples:
   • For the first 10 lines:

glow --head

• For the last 10 lines:

glow --tail=10

• For the first 5 lines:

glow --head=5

If it's tricky to implement, maybe a --color-always option could help keep syntax color when piping to head or tail.