Update base64 function to support older neovim versions.

Lua code taken from - http://lua-users.org/wiki/BaseSixtyFour
This ensures that older versions without vim.base64 also work smoothly.

lua/picvim.lua

@@ -12,6 +12,36 @@ if not uv then
 	uv = vim.loop
 end
 
+-- Lines 15-43 from http://lua-users.org/wiki/BaseSixtyFour
+local b='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/'
+function enc(data)
+    return ((data:gsub('.', function(x) 
+        local r,b='',x:byte()
+        for i=8,1,-1 do r=r..(b%2^i-b%2^(i-1)>0 and '1' or '0') end
+        return r;
+    end)..'0000'):gsub('%d%d%d?%d?%d?%d?', function(x)
+        if (#x < 6) then return '' end
+        local c=0
+        for i=1,6 do c=c+(x:sub(i,i)=='1' and 2^(6-i) or 0) end
+        return b:sub(c+1,c+1)
+    end)..({ '', '==', '=' })[#data%3+1])
+end
+function dec(data)
+    data = string.gsub(data, '[^'..b..'=]', '')
+    return (data:gsub('.', function(x)
+        if (x == '=') then return '' end
+        local r,f='',(b:find(x)-1)
+        for i=6,1,-1 do r=r..(f%2^i-f%2^(i-1)>0 and '1' or '0') end
+        return r;
+    end):gsub('%d%d%d?%d?%d?%d?%d?%d?', function(x)
+        if (#x ~= 8) then return '' end
+        local c=0
+        for i=1,8 do c=c+(x:sub(i,i)=='1' and 2^(8-i) or 0) end
+            return string.char(c)
+    end))
+end
+-- End of code from http://lua-users.org/wiki/BaseSixtyFour
+
 local Image = {}
 Image.__index = Image
 
@@ -41,7 +71,7 @@ function Image:draw(x, y, w, h)
 	end
 	local data = file:read("*all")
 	file:close()
-	local encoded_data = vim.base64.encode(data):gsub("%/", "/")
+	local encoded_data = enc(data)
 	local pos = 1
 	local chunk_size = 4096
 	stdout:write("\27[" .. x + 2 .. ";" .. y + 4 .. "H")