Add org-ql-plain-string-predicate and use it

README.org

@@ -74,6 +74,8 @@ These commands and functions are included:
      -  ~org-ql-select~ (function)
      -  ~org-ql-query~ (function)
 
+If user search bare string, the predicate ~org-ql-plain-string-predicate~ will be used by default.
+
 Feedback on these APIs is welcome.  Eventually, after being tested and polished, they will be considered stable. 
 
 # TODO: Find a way to get these examples included in the info manual.
@@ -378,6 +380,8 @@ Expands into a call to ~org-ql-select~ with the same arguments.  For convenience
 ** 0.4-pre
 
 *Added*
++  *Variable*
+     -  ~org-ql-plain-string-predicate~, which will be used when user search bare string.
 +  *Commands*
      -  ~helm-org-ql-views~, which shows one of ~org-ql-views~ selected with Helm.
      -  ~org-ql-search~ can search files in ~org-directory~; customization options are available in the ~org-ql-search~ group.