Introduce smart trimming

[fatfisz]

This should fix the long-standing issue #37.

The rules for "smart trimming" are as such:

• Each line in a string is treated separately, meaning that empty lines won't be removed (with a small exception).
• Whitespace is trimmed at the end of each line.
• The only empty line that will be removed is the very first one, and only if it's empty. That's beacuse of the common pattern of using backticks as parentheses:

  const multiLineText = stripIndent`
    foo
    bar
  `;

  In this case it's safe to assume that the first newline is not wanted in the end result.

• On the other hand the final newline will be left as-is (after trimming the preceeding whitespace). This seems to be a good default, and is actually a long-standing issue #37.

To sum up, smart trimming is about sane defaults and working together with indent-stripping tags.

[codecov[bot]]

Codecov Report

Merging #165 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #165   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     27           
  Lines         111    112    +1     
  Branches       29     29           
=====================================
+ Hits          111    112    +1

Impacted Files	Coverage Δ
src/stripIndent/stripIndent.js	100% <ø> (ø)	:arrow_up:
src/commaListsOr/commaListsOr.js	100% <ø> (ø)	:arrow_up:
src/commaLists/commaLists.js	100% <ø> (ø)	:arrow_up:
src/commaListsAnd/commaListsAnd.js	100% <ø> (ø)	:arrow_up:
src/html/html.js	100% <ø> (ø)	:arrow_up:
src/stripIndents/stripIndents.js	100% <ø> (ø)	:arrow_up:
src/safeHtml/safeHtml.js	100% <ø> (ø)	:arrow_up:
src/testUtils/readFromFixture/readFromFixture.js	100% <100%> (ø)	:arrow_up:
src/trimResultTransformer/trimResultTransformer.js	100% <100%> (ø)	:arrow_up:
src/inlineLists/inlineLists.js	100% <100%> (ø)	:arrow_up:

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