add spectre plugin support

[uloco]

create_highlight_group('SpectreHeader', "Comment")
create_highlight_group('SpectreBody', "String")
create_highlight_group('SpectreFile', "Keyword")
create_highlight_group('SpectreDir', "Comment")
create_highlight_group('SpectreSearch', "DiffChange")
create_highlight_group('SpectreBorder', "Comment")
create_highlight_group('SpectreReplace', "DiffDelete")

see: https://github.com/nvim-pack/nvim-spectre/blob/master/lua/spectre/highlight.lua

[uloco]

currently not themeable