Add find_element and get_elements_by_tagname methods

[vargonis]

1. It is proposed that find_element and get_elements_by_tagname accept, besides an XMLElement, also an XMLDocument as first argument.
2. A method find_element(x, ns::Vector{<:AbstractString}) is implemented. It just folds find_element over the vector of abstract strings.

[codecov[bot]]

Codecov Report

Merging #105 into master will decrease coverage by 2.45%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   76.97%   74.52%   -2.46%     
==========================================
  Files           6        6              
  Lines         152      157       +5     
==========================================
  Hits          117      117              
- Misses         35       40       +5

Impacted Files	Coverage Δ
src/nodes.jl	78.84% <0%> (-0.77%)	:arrow_down:
src/document.jl	75% <0%> (-9.38%)	:arrow_down:

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[omus]

Needs tests