Linter mode "manual" throwing an exception
Context:
macOS 10.13.5
Sublime Text 3.1.1 Build 3176
SublimeLinter v4.6.4
SublimeLinter-flake8 v4.1.0

Background:
When adding the setting `"lint_mode": "manual"` to my settings for SublimeLinter, I see an error. All other settings ("background". "load_save", and "save") do not produce an error.

Settings:
```
// SublimeLinter Settings - User
{
	"debug": true,
	"lint_mode": "manual",
	"linters": {
		"flake8": {
			"executable": "~/miniconda3/bin/flake8",
			"show-code": false
		},
	},
}
```


Error (from SublimeText console):
```
SublimeLinter: linter.py:798: Checking lint mode manual vs lint reason None
Exception in thread Thread-44:
Traceback (most recent call last):
  File "./python3.3/threading.py", line 901, in _bootstrap_inner
  File "./python3.3/threading.py", line 1142, in run
  File "<home>/Library/Application Support/Sublime Text 3/Installed Packages/SublimeLinter.sublime-package/sublime_linter.py", line 257, in lint
    linter for linter in linters
  File "<home>/Library/Application Support/Sublime Text 3/Installed Packages/SublimeLinter.sublime-package/sublime_linter.py", line 258, in <listcomp>
    if linter.should_lint(reason)]
  File "<home>/Library/Application Support/Sublime Text 3/Installed Packages/SublimeLinter.sublime-package/lint/linter.py", line 801, in should_lint
    return reason in _ACCEPTABLE_REASONS_MAP[lint_mode]
TypeError: 'in <string>' requires string as left operand, not NoneType
```
diff --git a/lint/linter.py b/lint/linter.py
index 4f94ae1..23e58c4 100644
--- a/lint/linter.py
+++ b/lint/linter.py
@@ -36,7 +36,7 @@ UTF8_ENV_VARS = {
 # how lint_mode is implemented. The map is supposed to be used in
 # this module only.
 _ACCEPTABLE_REASONS_MAP = {
-    "manual": ("on_user_request"),
+    "manual": ("on_user_request",),
     "save": ("on_user_request", "on_save"),
     "load_save": ("on_user_request", "on_save", "on_load"),
     "background": ("on_user_request", "on_save", "on_load", None),
['lint/linter.py']