Enable substring autocompletion for `/msg`

[jubalh]

People with huge contact lists most likely happen to have contacts starting with popular names more often. They can use /msg Arthur to find the correct person.

But if they have 50 Arthurs it will take a while. Using the surname might be faster. So a substring search of /msg Clarke<tab> will be faster.

The question is if we want this as a general behaviour for roster contact completion or just for /msg? Opinions?