add keyword arguments to var and std
Merge branch 'master' into dh/cov2
new covm and cov implementations
cov tested
import REPL.jl code

Fixes #1773 (Repl autocompletion for composite type fields).
Fixes #3652 (Segfault during TAB-completion on OSX).
Fixes #4477 (Tab completion for `using` should look up installed packages).
integrate REPL.jl code into Base
expunge readline
fix repl crash when tab-completing type field
fix repl crash when pasting trailing newlines
add old-repl path completion behavior to new-repl

Cf. #5343
fix buffer size bug in Readline.edit_replace

A symptom of this is if

    ext(c)<left><left><left><tab><enter>

is entered into a fresh session, the output is

    extrema (generic function with 2 methods)

instead of the expected

    ERROR: c not defined
use unscaled_cov as core function
simplify the interface of covm and cov
add new corm and cor methods
repl: fix color detection on windows
repl: bind Delete key
repl: don't ignore --no-history option

Also, hoist all the using/imports to the top.
cor & corm tested
export covm and corm
optimized implementation of unscaled_covzm