Add search option to apply term boosting

[lucaong]

Term boosting (giving greater or lower importance to specific query terms) was previously not supported. It was technically possible by using the boostDocument search option (as shown here: #268) but cumbersome and error prone.

This commit adds a new search option, boostTerm, which makes it a lot easier to apply term boosting. The option is a function that is invoked with each query term (as normalized by processTerm), as well as the term index in the query, and the array of all query terms, and is expected to return a numeric boost factor.

A factor greater than 1 increases the importance of a term for the relevance scoring, a factor less than 1 decreases the importance of a term, and a factor equal to 1 is neutral.

Examples

Boosting specific terms:

const boostFactors = {
  'motorcycle': 1.2,
  'foo': 0.7
}

miniSearch.search('Motorcycle Maintenance', {
  boostTerm: (term) => boostFactors[term] || 1
})

Boosting the first few terms in the query:

const boostFactors = [1.5, 1.2]

miniSearch.search('Motorcycle Maintenance', {
  boostTerm: (term, i) => boostFactors[i] || 1
})