Merge pull request #30 from vasily-kirichenko/master

fix: single-upper-case symbols are not highlighted
Restrict symbol lookups on identifiers only

Use a line tokenizer to be able to recognize strings, comments and
compiler directives.

Fix #22.
Fix #24.
Highlight operators

Fix #26.

No longer highlight symbols if we can't find their references (e.g.
namespaces, type annotations, etc.).
Use "Highlight References" consistently throughout the solution
fixed: operators that started with '>' symbol are not highlighted