generate dictionary/omnifunc completion database on demand

[drichardson]

Since this plugin can be used with any version of Houdini, it should be able to generate the dictionary/omnifunc completion database on demand in case new VEX functions are added.

[drichardson]

If started as an external editor from Houdini, the HFS environment variable will be set. Could use this to select/write to a Houdini version specific completion database.

For example, suppose you're running Houdini 17.5.229 and you launch Vim as your external editor for the first time after installing vex.vim, vex.vim would:

1. Look for a file called database-17.5.229.txt. If found, use it and stop.
2. If not found, generate the database using vcc found at $HFS/bin/vcc, where $HFS is the Houdini install environment variable.

If not started from Houdini, need to figure out a way to select the correct context for completion. That will be addressed in another issue.