Add some vader tests for completion feature

I want to improve file/path completion, so add some tests to cover
existing behaviour (at least existing behaviour as I understand it).

Author: mmrwoods

test/feature/completion.vader

@@ -0,0 +1,57 @@
+Before:
+  enew!
+  only!
+  cd include/completion
+  let w:testing = 1
+
+After:
+  cd -
+
+Execute (flags, match all):
+  Assert len(grepper#complete('-', '', v:null)) > 3
+  AssertEqual count(grepper#complete('-', '', v:null), '-cword '), 1
+  AssertEqual count(grepper#complete('-', '', v:null), '-grepprg '), 1
+  AssertEqual count(grepper#complete('-', '', v:null), '-tool '), 1
+
+Execute (flags, match one):
+  AssertEqual grepper#complete('-noquickf', '', v:null), ['-noquickfix ']
+
+Execute (flags, match some):
+  Assert len(grepper#complete('-no', '', v:null)) > 2
+  AssertEqual count(grepper#complete('-no', '', v:null), '-switch '), 0
+  AssertEqual count(grepper#complete('-no', '', v:null), '-noswitch '), 1
+
+Execute (flags, -dir options):
+  Assert len(grepper#complete('', 'Grepper -dir ', v:null)) > 2
+  AssertEqual count(grepper#complete('', 'Grepper -dir ', v:null), 'cwd '), 1
+  AssertEqual count(grepper#complete('', 'Grepper -dir ', v:null), 'repo '), 1
+
+Execute (flags, -stop completion):
+  Assert grepper#complete('', 'Grepper -stop ', v:null)[0] =~# '\d\+'
+
+Execute (flags, -tool options):
+  Assert len(grepper#complete('', 'Grepper -tool ', v:null)) > 1
+  AssertEqual count(grepper#complete('', 'Grepper -tool ', v:null), 'grep '), 1
+
+Execute (relative path, empty string):
+  AssertEqual grepper#complete('', '', v:null), ['./foo/']
+
+Execute (relative path, whitespace only):
+  AssertEqual grepper#complete('  ', '', v:null), ['  ./foo/']
+
+Execute (sub-path, ./ exact match):
+  AssertEqual grepper#complete('./', '', v:null), ['./foo/']
+
+Execute (sub-path, ./ and word chars):
+  AssertEqual grepper#complete('./f', '', v:null), ['./foo/']
+
+Execute (nested sub-path, e.g. ./foo/):
+  AssertEqual grepper#complete('./foo/', '', v:null), ['./foo/bar/']
+
+Execute (absolute path, / exact match):
+  AssertEqual count(grepper#complete('/', '', v:null), '/bin/'), 1
+  AssertEqual count(grepper#complete('/', '', v:null), '/var/'), 1
+
+Execute (absolute path, / and word chars):
+  AssertEqual count(grepper#complete('/b', '', v:null), '/bin/'), 1
+  AssertEqual count(grepper#complete('/b', '', v:null), '/var/'), 0