Double-click selection matches some escaped characters

Issue Type: <b>Bug</b> If you have a string with the "\t", "\n", etc. sequences followed by a word, double-clicking the word will select the escaped character as well:  VS Code version: Code 1.21.0 (9a199d77c82fcb82f39c68bb33c614af01c111ba, 2018-03-07T11:01:43.521Z) OS version: Linux x64 4.10.0-42-generic <details> <summary>System Info</summary> |Item|Value| |---|---| |CPUs|Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz (8 x 3142)| |Load (avg)|0, 1, 1| |Memory (System)|15.56GB (3.82GB free)| |Process Argv|/usr/share/code/code --unity-launch| |Screen Reader|no| |VM|0%| </details><details><summary>Extensions (7)</summary> Extension|Author (truncated)|Version ---|---|--- vscode-icontheme-nomo-dark|be5|1.3.5 vscode-eslint|dba|1.4.7 githistory|don|0.4.0 auto-close-tag|for|0.5.6 docthis|joe|0.6.0 python|ms-|2018.2.0 color-highlight|nau|2.3.0 (1 theme extensions excluded) </details> Reproduces without extensions <!-- generated by issue reporter -->