feat: Explicitly set marker for sexy comments

[albfan]

Instead of extract the marker from last char of longest left comment allow to explicity set it

relates with #329

[albfan]

to turn:

  <metadata>
    <property name="variant">dark</property>
    <property name="light-variant">Adwaita</property>
  </metadata>

into

  <!-- <metadata>
    <property name="variant">dark</property>
    <property name="light-variant">Adwaita</property>
  </metadata> -->

instead of

  <!-- <metadata>
  -   <property name="variant">dark</property>
  -   <property name="light-variant">Adwaita</property>
  - </metadata> -->

using:

let g:NERDCustomDelimiters = {
        \ 'xml':  { 'left': '<!--', 'right': '-->', 'marker': '', 'nested': 1 }
    \ }

[albfan]

NOTE1: 'nested' is required to avoid an error:

        call s:NerdEcho('Sexy comment aborted. Nested sexy cannot be nested', 0)

NOTE2:
Expectation was to only need to set:

let g:NERDCustomDelimiters = {
        \ 'xml':  { 'marker': '' }
    \ }

But that doesn't do anything using .cs

[alerque]

Resolve case where marker is not set.

[alerque]

It doesn't look like this buffer value is getting set unconditionally, so this will error in the event it is not set ... no?