Support for triggers in ContentIndexParser

[geoo89]

In the content index sheet, the relevant row type is create_triggers. The sheet name must reference a valid sheet.

This sheet has 4 important columns (also see here):

• type: K (keyword), C (uncaught) or M (missed call)
• keyword: keyword in case the type is K
• flow: Name of the flow that gets triggered
• groups: List of group names for which the trigger applies (may be empty)

Issues:

• RapidPro's UI supports other triggers, like starting a flow at a given point in time, and a few others, however, none of them seem to work (I was unable to create them, the UI seems buggy).
• Similarly, for keyword triggers, the UI allows to check whether the user's message starts with the keyword or is exactly the keyword, however, during export, this information is lost (these is no field in the json to distinguish between the two).
• There is also a channel field in the UI and the exported JSON, however, I was unable to get it to work in the UI. It is supported as a column in the trigger sheet, and its type is assumed to be string, but I don't know if it works or has any effect. Leave it blank (or leave it out of the column headers altogether) to make sure that its value is set to null.

Fixes #92

[fagiothree]

@geoo89 from what I could see in an exported file from the latest RapidPro version, for triggers of type K, there is a field called "match_type" :
"O" --> matches
"F" --> starts with