Return boolean in goBack function

[pbogut]

Returns boolean flag in goBack function that is true if can go back and false if it can't.

Reason

I'm using <bs> for my for a file browser (dirvish), but I also prefere to have it mapped to goBack when in markdawn files.
With this change I can set it up so I have my file browser opened when I cant go back. Works perfect for me and this change should not have any effect for existing users.

If you don't like this solution then maybe I can add public method canGoBack and use that in my mapping?

My mapping for reference:

if not require('mkdnflow').files.goBack() then
  vim.cmd('Dirvish %:p')
end

[jakewvincent]

@pbogut Thanks for the PR! I'll test this out shortly--AFAIK it shouldn't hurt anything.