Add Markdown to "scope_ignore" default setting

[darkred]

This PR adds Markdown to the "scope_ignore" default setting. The reason is because trimming trailing spaces changes the output of the Markdown (2 trailing spaces means <br>).

Also, for reference, currently, in README.me the "scope_ignore" reference has:

// Trailing spaces for find results, build output and markdown are ignored
{ "scope_ignore": ["text.find-in-files", "source.build_output", "text.html.markdown"] }

while in the actual trailing_spaces.sublime-settings file currently is:

// By default, any lines in the Find Results Build output and Diff views are ignored
// Add scopes to this list if you need to ignore them.
"scope_ignore": ["text.find-in-files", "source.build_output", "source.diff"],

[rchl]

Also, for reference, currently, in README.me the "scope_ignore" reference has:

It has correct value on the line that starts with default *Default

But in any case, could you also update the readme? :)

[darkred]

Yes, I just did (and added it on the line with the *Default too).

[darkred]

Thanks for the merge.