Add invert keyword feature

[mlsteele]

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Description

Add an option for monitors to look for the absence rather the the presence of a keyword. For example to make sure a response does not include the phrase "error".

My motivation is monitoring a the alarm status of a Sensaphone device. Which outputs an xml page that, in the absence of parsing, has many occurrences of the string "Good". And then when there's a problem one of the occurrences changes to "Bad". "Upside down" mode is not suited to the task because the monitor should alert if the page is unreachable.

Type of change

• User interface (UI)
• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I ran ESLint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
  (including JSDoc for methods)
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)

Screenshots (if any)

[mlsteele]

@louislam wdyt?
I'd like to get back to running main instead of a fork but need this feature.

[louislam]

@louislam wdyt? I'd like to get back to running main instead of a fork but need this feature.

Thanks for the pr. Tested, it is working.

It may be a bit wired when both Upside Down mode + Invert Keyword are checked, but never mind, we could think about this later.