Update MatchParen in light themes.

In order to address

    https://github.com/lifepillar/vim-solarized8/issues/14

and further concerns in

    https://github.com/lifepillar/vim-solarized8/issues/15

in the previous commit and in this one I have updated Cursor and
MatchParen highlighting, respectively.

The current colors are based on the following considerations:

1. The Solarized palette has only two "whites": the ligther is used in
   all variants but "low", which uses the darker.

2. Matching parentheses should be distinguishable even if bold face is
   disabled.

3. The cursor and matching parentheses should be easily distinguishable
   even when 'cursorline' is on. Before the recent commits this was not
   the case.

4. MatchParen's background may be set to NONE in the "low" and "flat"
   variants, because CursorLine uses underline. It should be set to
   something different in the other two variants, otherwise it may
   become impossible to see a matching character.

5. MatchParen's foreground color shouldn't be red if MatchParen's background
   color is not set to something different from the normal background.
   This is because several syntax files use red to highlight parentheses
   by default. If bold fonts are not used, matching parentheses may be
   not be visible at all.

6. I don't like red on grey (see MatchParen in original Solarized) or
   red next to grey.

So, these are the choices I've made:

- In the normal and high contrast variant the cursor is now orange and
  MatchParen's background is grey (with white foreground, see 6 above).
  This achieves the twofold goal of being able to see a matching
  parenthesis when 'cursorline' is on and being able to clearly
  distinguish the cursor from the matching parenthesis. Orange is
  preferred to red because of 6 (with two consecutive parentheses
  you have a grey block and an orange block next to each other).

- In the flat and low contrast variant the cursor is grey, exactly as in
  the original Solarized. This choice is made because such variants
  should have less contrast in general, and because 'cursorline' poses
  no problem as it does not change the text background.

- The flat variant uses the colors that were used previously for all the
  variants. Even without boldface, the matching parenthesis can be seen
  because it has a slightly darker background (see 1).

- The low contrast variant uses the darker white for the normal
  background, so MatchParen has a transparent background and a nearly
  black foreground. This does not particularly stand out without
  boldface, but hey, it's low contrast.

colors/solarized8_light_high.vim

@@ -138,7 +138,7 @@ if has('nvim')
   let g:terminal_color_14 = '#93a1a1'
   let g:terminal_color_15 = '#fdf6e3'
 endif
-hi! MatchParen cterm=NONE,bold gui=NONE,bold ctermfg=1 guifg=#dc322f ctermbg=7 guibg=#eee8d5
+hi! MatchParen cterm=NONE,bold gui=NONE,bold ctermfg=15 guifg=#fdf6e3 ctermbg=11 guibg=#657b83
 hi! link vimVar Identifier
 hi! link vimFunc Function
 hi! link vimUserFunc Function