Make editor local.

[bjornbm]

A benefit of making editor local would be that there is no risk of:

• accidentally using it in, for example, buffer.lua (I'm sure you wouldn't but maybe a well-meaning patch might…), or
• something silly like ple_init setting editor = nil breaking the editor (which now will cause the below but with 933b2b3 does not crash).

/Users/bjorn/repos/ple/ple.lua:220: attempt to index a nil value (global 'editor')                                              
stack traceback:
	/Users/bjorn/repos/ple/ple.lua:220: in field 'statusline'
	/Users/bjorn/repos/ple/ple.lua:348: in upvalue 'adjcursor'
	/Users/bjorn/repos/ple/ple.lua:391: in upvalue 'redisplay'
	/Users/bjorn/repos/ple/ple.lua:416: in field 'fullredisplay'
	/Users/bjorn/repos/ple/ple.lua:797: in field 'newbuffer'
	/Users/bjorn/repos/ple/ple.lua:1085: in function </Users/bjorn/repos/ple/ple.lua:1080>
	[C]: in function 'xpcall'
	/Users/bjorn/repos/ple/ple.lua:1133: in local 'main'
	/Users/bjorn/repos/ple/ple.lua:1147: in main chunk
	[C]: in function 'dofile'
	/Users/bjorn/bin/ple:5: in main chunk
	[C]: in

Supersedes #10.

[bjornbm]

This is a bit wasted effort in case no initfile is found. Also there is some duplication on the lines below (1056, 1060, 1065).

Let me know if you prefer to also separate the locating and the loading of the initfile as in 8280237 (addresses the wasted effort and duplication).