Support for nested schema

[DanieleFedeli]

This is the PR for the issue #14

Coverage pre PR:

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	86.44	68.13	90	87.14
src	82.43	58.18	84.61	82.38
errors.ts	82.6	100	50	75	8,20,23,26
levenshtein.ts	91.66	60	100	100	2-3
lyra.ts	89.71	65.62	100	89.71	76-81,113,150-153,168,179,238,270
stemmer.ts	30.76	18.18	100	30.76	22-23,28-49,56
tokenizer.ts	100	100	100	100
utils.ts	94.73	83.33	100	94.73	30
src/prefix-tree	95.55	83.33	100	97.7
node.ts	100	100	100	100
trie.ts	94.59	83.33	100	97.18	121,146

Coverage post PR:

File	% Stmts	% Branch	% Funcs	% Lines	Uncovered Line #s
All files	87.95	72.82	90	89.04
src	84.68	66.07	84.61	85.2
errors.ts	82.6	100	50	75	8,20,23,26
levenshtein.ts	91.66	60	100	100	2-3
lyra.ts	93.69	78.78	100	94.54	114,151-154,169,180
stemmer.ts	30.76	18.18	100	30.76	22-23,28-49,56
tokenizer.ts	100	100	100	100
utils.ts	94.73	83.33	100	94.73	30
src/prefix-tree	95.55	83.33	100	97.7
node.ts	100	100	100	100
trie.ts	94.59	83.33	100	97.18	121,146

[micheleriva]

Beautiful! LGTM