Added new target file extension of encryption, ".htm", with consideration of capital...

Added new target file extension of encryption, ".htm", with consideration of capital letters.

[Issue#]   LINUXWRT-983
[Problem]  Some html files included in encryption widget are not encrypted after installation.
[Cause]    wrt-installer decides target files of encryption by refering to each filename extension.
           However, ".htm" was omitted in target list.
[Solution] Added new extension define, ".htm", for encryption.
           Additionally, consideration of capital letter was also added.

Change-Id: If6efdf3f54ead6085631bdd37fec1e23ae10b7ee