Make INSTALLLEVEL and REBOOT properties configurable

[HKrausAxon]

As with other properties in the wix template, it can
be useful for users of this library to override both
the reboot behavior (e.g. in cases they want to allow
reboots), as well as the install level (e.g. if they
want to exclude install level 2 features or include
install level 3 features without customers having to
use ADDLOCAL or other manual mechanisms).

This change makes both of those properties configurable,
defaulting them to their existing behavior.

It also cleans up a leftover comment (there were two sets
of descriptions for INSTALLPATH; this change removes the
older, misplaced version.

Screenshots

msi created after this change, without setting any custom configs:

--> We can see INSTALLLEVEL is set to 2, and REBOOT is set to "ReallySuppress"

In this attempt, I added the following two lines to my MSICreator config:

	rebootMode: 'Suppress',
	installLevel: 3,

Which resulted in the following msi properties:

--> We can see, as expected INSTALLLEVEL is set to 3 and REBOOT is set to "Suppress"
I also validated that Update.exe gets installed and folder permissions are set as per expectation when INSTALLLEVEL is set to "3" (in accordance with the "Auto Update" feature)

[coveralls]

Coverage remained the same at 100.0% when pulling c626aa1 on HKrausAxon:add_configurability into 4bfb056 on felixrieseberg:master.

[HKrausAxon]

Note: The failing tests are unrelated to this PR and had been failing beforehand.
I have created a separate PR to fix those existing issues: #117

[felixrieseberg]

This looks like a good addition, thank you! I'll merge the PR and release a new minor version.