Show mapping only for localleader

[ghost]

Is your feature request related to a problem? Please describe.
I have set leader to <Space> and localleader to <Space>m.
I would like to show local mappings only for the buffers for which those mappings exists.
For example I have the following autocommand:

autocmd FileType markdown nnoremap <buffer> <localleader>p :MarkdownPreview<CR>

I could use the following

let g:which_key_map['m']['p'] = 'preview'

however, that would show that mapping for every buffer, even not markdown files.

Describe the solution you'd like
To be able to show mappings only for specific files

[liuchengxu]

Duplicate as #48, but I haven't worked on that for a while.