Feature request: helm-initial-inputs-alist

**Is your feature request related to a problem? Please describe.**
For a handful of functions, I wind up typing `^` quite often into the minibuffer.

**Describe the solution you'd like**
Some sort of `defcustom`, `helm-initial-inputs-alist` to allow the user to specify an initial input in the `helm` minibuffer for a function. Ivy already has this.

Start by examining Helm's minibuffer entry points and compare Ivy's existing initial-input behavior. Determine how a defcustom named helm-initial-inputs-alist should map functions to initial input, then verify that the requested functions begin with the configured text without affecting other Helm prompts.

Written by the indexing model from the issue text.