feat: support configuration via vim.g.render_markdown_config

## Details

Users can now optionally configure this plugin using the
`render_markdown_config` global. The value of this global is the same as
what a user would normally pass to the `setup` method. Calling `setup`
manually is not required when using this configuration method.

Using this method of configuration provides a guarantee that the correct
configuration will always be used, which is only really a problem when
it comes to various lazy loading mechanisms that can potentially use the
default configuration for the first buffer opened.

Author: MeanderingProgrammer

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*       For NVIM v0.11.4      Last change: 2025 October 28
+*render-markdown.txt*      For NVIM v0.11.5      Last change: 2025 November 03
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/health.lua

@@ -6,7 +6,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.9.12'
+M.version = '8.9.13'
 
 function M.check()
     M.start('versions')

plugin/render-markdown.lua

@@ -4,7 +4,7 @@ if initialized then
 end
 initialized = true
 
-require('render-markdown').setup()
+require('render-markdown').setup(vim.g.render_markdown_config)
 require('render-markdown.core.colors').init()
 require('render-markdown.core.command').init()
 require('render-markdown.core.log').init()