Added forest_night theme (#30)

* Add forest_night theme

Ported from forest-night's [lightline
theme](https://github.com/sainnhe/forest-night/blob/master/autoload/lightline/colorscheme/forest_night.vim).

* Add forest_night to THEMES.md

THEMES.md

@@ -8,6 +8,12 @@ All available themes are only best effort ports by myself/ other users. If you f
 ![visual](https://user-images.githubusercontent.com/41551030/103468264-652eee00-4d57-11eb-8ec8-e433cb396a63.png)
 ![replace](https://user-images.githubusercontent.com/41551030/103468272-88f23400-4d57-11eb-93fe-c3715f1bc335.png)
 
+### forest_night
+![Normal](https://user-images.githubusercontent.com/10103146/103585856-b8f52080-4e98-11eb-85df-1c62edcf5d04.png)
+![Insert](https://user-images.githubusercontent.com/10103146/103585855-b85c8a00-4e98-11eb-8abd-9d99b9d2be58.png)
+![Visual](https://user-images.githubusercontent.com/10103146/103585852-b7c3f380-4e98-11eb-9022-59deaf804c81.png)
+![Replace](https://user-images.githubusercontent.com/10103146/103585857-b8f52080-4e98-11eb-9e71-b34ddb9612bc.png)
+
 ### gruvbox
 ![normal](https://user-images.githubusercontent.com/41551030/103467902-06b44080-4d54-11eb-89db-6d3bebf449fa.png)
 ![insert](https://user-images.githubusercontent.com/41551030/103467914-1764b680-4d54-11eb-9e3d-528d3568dce7.png)

lua/lualine/themes/forest_night.lua

@@ -0,0 +1,121 @@
+local forest_night = {}
+
+local colors = {
+  bg0 = "#323d43",
+  bg1 = "#3c474d",
+  bg3 = "#505a60",
+  fg = "#d8caac",
+  aqua = "#87c095",
+  green = "#a7c080",
+  orange = "#e39b7b",
+  purple = "#d39bb6",
+  red = "#e68183",
+  grey1 = "#868d80",
+}
+
+forest_night.normal = {
+  a = {
+    bg = colors.green,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.insert = {
+  a = {
+    bg = colors.fg,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.visual = {
+  a = {
+    bg = colors.red,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.replace = {
+  a = {
+    bg = colors.orange,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.command = {
+  a = {
+    bg = colors.aqua,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.terminal = {
+  a = {
+    bg = colors.purple,
+    fg = colors.bg0,
+  },
+  b = {
+    bg = colors.bg3,
+    fg = colors.fg,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.fg,
+  },
+}
+
+forest_night.inactive = {
+  a = {
+    bg = colors.bg1,
+    fg = colors.grey1,
+  },
+  b = {
+    bg = colors.bg1,
+    fg = colors.grey1,
+  },
+  c = {
+    bg = colors.bg1,
+    fg = colors.grey1,
+  },
+}
+
+return forest_night