Correct tango color scheme for terminals
Update vimrc to always set colour scheme

If syntax was already enabled the tango colour scheme would not be set.
Move `colors tango` into a new if statement to correct this.
Update tango colorscheme to underline spelling

Use `gui=undercurl` on the spell groups instead of completely changing
all syntax highlighting for spelling mistakes. This change is only
applicable to gvim.
Merge branch 'tango_update'
Simplify file detection for .text files