company-ycmd: When disabling fuzzy matching omit non-prefix candidates

When setting company-ycmd-enable-fuzzy-matching to nil, do not add
non-prefix candidates to candidates list. Add also a new defcustom to
control company-mode's no-cache feature.
Merge pull request #111 from ptrv/disable-enable-fuzzy-matching

company-ycmd: When disabling fuzzy matching omit non-prefix candidates